METHODS AND SYSTEMS FOR HYPOXIA TRAINING FIELD OF THE INVENTION 5 The present invention relates to methods and systems for the training or treatment of an animal subject using the inhalation of hypoxic and optionally hyperoxic air. In particular, the invention provides methods and systems allowing for optimising a health effect or a performance effect of hypoxia and optionally hyperoxia. .0 BACKGROUND TO THE INVENTION It is known in the art that the health of an individual can be improved by the breathing of hypoxic air (i.e. air containing less than normal atmospheric levels of oxygen) for short periods of time. It has been found that the breathing of hypoxic air is useful in the treatment .5 or prevention of conditions including chronic heart and lung disease, hypertension, asthma and chronic bronchitis, liver and pancreatic diseases, anxiety and depression, anaemia due to iron-deficiency, lack of energy and fatigue. It is further known in the art that short periods of hypoxia can be used to improve aerobic 0 and anaerobic performance of an individual. For example, hypoxic training has been shown to increase power at both anaerobic threshold and V0 2 max by 4-6% in as little as three weeks with a daily session for an hour at a time. For both health and performance effects, hypoxia is typically effected in the form of 25 intermittent hypoxic training (IHT). An IHT session may consists of an interval of several minutes breathing hypoxic air, alternated with intervals breathing ambient or hyperoxic air. This procedure is typically repeated over a 45- to 90-minute session per day, with a full treatment course taking three to four weeks. Where hyperoxic air is included, the method is generally termed intermittent hypoxic hyperoxic training (IHHT). 30 Standard practice is for the subject to remain stationary during the session while breathing the hypoxic air via a face mask. A software-controlled hypoxia machine is often used whereby an automated session of IHT or IHHT is administered whereby the levels of oxygen (and also the period for which air of a given oxygen level is administered) is pre 35 programmed. -1- Many approaches are used to assess the effectiveness of an IHT or IHHT treatment program. For example, a clinical endpoint according to the condition for which the subject is treated may be utilized in an assessment of efficacy. For example, a decrease in blood [0 pressure of a certain percentage would be a useful clinical endpoint where the subject is treated for hypertension. When used in the context of athletic training, a performance endpoint such as power output for the subject may be utilized. As another example, pulse oximetry to determine arterial oxygen saturation (SpO 2 ) may be [5 used to ensure that a predetermined level of hypoxia is achieved during a treatment session. Due to the sigmoidal profile of the oxyhaemoglobin dissociation curve, tissue hypoxia develops only when SpO2 drops to 90% or below. Accordingly, the oxygen concentration in the inhaled air may need to be further adjusted downwardly where the SpO 2 of a subject does not reach a target level. In some IHT and IHHT methods, a biofeedback loop is io included whereby the oxygen concentration of inhaled air is automatically modulated in response to the SpO 2 of the subject. A more standardised approach to the assessment of the effect of IHT or IHHT is that of the Hypoxic Training Index (HTi). This index is based on SpO 2 , but provides an objective 15 measure of the hypoxic stress delivered during an IHT or IHHT session, compared to simple recording the inhaled fraction of oxygen (FiO2). HTi provides a figure (index) of dosage received by the individual at the end of the session. Knowledge of HTi can be used to alter the training regime for different individuals, compensating for individual variability, and can be used in scientific studies to ensure that subject exposure was correctly controlled. iO While clinical endpoints, performance endpoints, SpO 2 and HTi are all useful in the design and assessment of an IHT or IHHT treatment session or regime, in some individuals at least a less than desired or required outcome is noted. The causes behind suboptimal outcomes is unclear, and the quest for improved IHT and IHHT methods is ongoing. 65 Accordingly, it is an aspect of the present invention provide systems and methods for improving individual outcomes in IHT and IHHT. It is a further aspect to provide an alternative to prior art IHT and IHHT systems and methods. 70 The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or -2were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. '5 SUMMARY OF THE INVENTION In a first aspect, but not necessarily the broadest aspect, the present invention provides a method for treating a subject using intermittent hypoxia, the method comprising the steps of: i0 exposing the subject to hypoxic air for a time period, measuring the heart rate variability of the subject, and altering the oxygen concentration in the hypoxic air and/or the time period for which the subject is exposed to the hypoxic air according to the measured heart rate variability. 15 In one embodiment of the method, the heart rate variability is measured at a plurality of time points over a single treatment session. In one embodiment of the method, the oxygen concentration in the hypoxic air and/or the )0 time period for which the subject is exposed to the hypoxic air heart rate variability is altered at a plurality of time points over a treatment session. In one embodiment of the method, the heart rate variability is measured at a plurality of time points over a single treatment session, and the oxygen concentration in the hypoxic air )5 and/or the time period for which the subject is exposed to the hypoxic air is altered at the plurality of time points over the single treatment session. In one embodiment of the method, the oxygen concentration in the hypoxic air and/or the time period for which the subject is exposed to the hypoxic air heart rate variability is altered 00 such that a predetermined positive or negative change in heart rate variability is achieved, or a predetermined heart rate variability is achieved. In one embodiment of the method, the oxygen concentration in the hypoxic air and/or the time period for which the subject is exposed to the hypoxic air heart rate variability is altered 05 such that a positive change in heart rate variability is achieved. In one embodiment the method comprises two or more treatment sessions, wherein the measured heart variability from a treatment session is used to set an oxygen concentration -3in the hypoxic air and/or the time period for which the subject is exposed to the hypoxic air .0 for a subsequent treatment session. In one embodiment the method comprises the step of exposing the subject to hyperoxic air. In a second aspect, there is further provided by the present invention a system for treating a .5 subject using intermittent hypoxia, the system comprising: a gas delivery means configured to provide a hypoxic air, and a heart rate variability measuring means, an automated control means configured to control the oxygen concentration in the hypoxic air and/or the time period for which a subject is exposed to the hypoxic air 0 according to the measured heart rate variability. In one embodiment of the system, the heart rate variability measuring means is configured to automatically measure heart rate variability at a plurality of time points over a single treatment session. 5 In one embodiment of the system, the gas delivery means is configured to automatically alter the oxygen concentration in the hypoxic air and/or the time period for which the subject is exposed to the hypoxic air heart rate variability at a plurality of time points over a single treatment session. io In one embodiment of the system, the heart rate variability measuring means is configured to automatically measure heart rate variability at a plurality of time points over a single treatment session, and the gas delivery means is configured to automatically alter oxygen concentration in the hypoxic air and/or the time period for which the subject is exposed to the 35 hypoxic air at the plurality of time points over the single treatment session. In one embodiment the system comprises an intra-session algorithm configured to instruct the controller to set or adjust the oxygen concentration of the hypoxic air according to the measured heart rate variability within a treatment session. 40 In one embodiment the system comprises an electronic memory configured to store a measured heart variability, and optionally the oxygen concentration of a delivered hypoxic gas, of a treatment session. -4- [5 In one embodiment the system comprises an inter-session algorithm configured to instruct the controller to set or adjust the oxygen concentration of the hypoxic air for a treatment session according to the stored heart rate variability, and optionally the oxygen concentration of a delivered hypoxic gas, in respect of a previous treatment session. io In one embodiment of the system, the controller is a processor-based device, the gas delivery means is controllable by the processor based device, and the heart rate variability measuring means is readable by the processor-based device; and wherein the processor based device is in operable communication with the gas delivery means and the heart rate variability means. 15 In one embodiment of the system, the heart rate variability measuring means is a heart rate measuring means in combination with the processor-based device. The present invention further provides in another aspect processor-executable software i0 configured to be operable within the system as described herein. In one embodiment the software comprises (i) an intra-session algorithm configured to instruct the controller to set or adjust the oxygen concentration of the hypoxic air according to the measured heart rate variability within a treatment session and/or (ii) inter-session i5 algorithm configured to instruct the controller to set or adjust the oxygen concentration of the hypoxic air for a treatment session according to the stored heart rate variability, and optionally the oxygen concentration of a delivered hypoxic gas, in respect of a previous treatment session. 70 A further aspect comprises a processor-based device comprising the software as described herein. BRIEF DESCRIPTION OF THE DRAWINGS 75 FIG. 1 shows a block diagram of a preferred system of the present invention. DETAILED DESCRIPTION OF THE INVENTION After considering this description it will be apparent to one skilled in the art how the invention 80 is implemented in various alternative embodiments and alternative applications. However, -5although various embodiments of the present invention will be described herein, it is understood that these embodiments are presented by way of example only, and not limitation. As such, this description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention. Furthermore, statements of 15 advantages or other aspects apply to specific exemplary embodiments, and not necessarily to all embodiments covered by the claims. Throughout the description and the claims of this specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude other )0 additives, components, integers or steps. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the )5 phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. In a first aspect the present invention provides a method for treating a subject using intermittent hypoxia, the method comprising the steps of: )0 exposing the subject to hypoxic air for a time period, measuring the heart rate variability of the subject, and altering the oxygen concentration in the hypoxic air and/or the time period for which the subject is exposed to the hypoxic air according to the measured heart rate variability. 05 Applicant has found that a subject having a condition associated with heart rate variability, or a subject desiring improvement in a performance parameter associated with the heart rate variability is assisted by IHT and IHHT. In particular, Applicant proposes the utility of heart rate variability as a biofeedback parameter within IHT or IHHT treatment sessions, or 10 between IHT or IHHT treatment sessions. This discovery provides for the first time the ability to design, redesign, improve, optimize, or personalize IHT or IHHT methods by reference to heart rate variability. This approach may provide means to identify treatments that (i) increase heart rate variability (that being a useful aim in itself as a general predictor of survival in some individuals) or (ii) treatments for conditions or performance issues that are 15 associated with low or high heart rate variability. -6- Conditions associated with low heart rate variability include death (given that reduced heart rate variability is a predictor of death after myocardial infarction, susceptibility to sudden infant death syndrome, and survival in premature infants), congestive heart failure, diabetic 0 neuropathy, organ transplant rejection, depression, anxiety, and post-traumatic stress disorder, and over training in athletes. Performance issues associated with heart rate variability include cardiorespiratory fitness (where a decrease variability in heart rate is generally required), endurance, power and the 15 like. While the prior art has suggested associations between heart rate variability and numerous conditions and performance parameters, before the priority date of the present application heart rate variability has not been proposed as a biofeedback parameter in IHT or IHHT as a !O means for overcoming a condition or a performance issue. Put another way, heart rate variability has only previously been considered to be a useful physiological marker only, with there being no suggestion of a use accordingly to the present invention. The present method is for the treating a subject using intermittent hypoxia. As used herein, 15 the term "treating" is intended to be construed broadly to mean not only a clinical treatment (i.e. to address a clinical condition) but also a treatment for other reasons including the improvement of physical or mental performance, physical appearance, general feeling of wellbeing, mental state, and the like. [0 The term "intermittent hypoxia" is intended to mean an intentionally administered program whereby a subject is exposed to hypoxic air for a period, followed by period of exposure to normoxic air (or hyperoxic air, in which case the term intermittent hypoxia hyperoxia may be used). An IHT session often consists of an interval of several minutes breathing hypoxic air, alternated with intervals breathing ambient or hyperoxic air. This procedure is repeated over 45 a 45- to 90-minute session per day, with a full treatment course often extending over three to four weeks The present methods require the measurement of heart rate variability (also known in the art as "cycle length variability" and "RR variability"). Hear rate variability arises due the innate 50 ability of the body to alter heart rate in accordance with physiological need. The SA node of the heart receives various electrical and chemical inputs and the resultant heart rate or RR interval and its variation are the results of these inputs. -7- The main inputs are the sympathetic and the parasympathetic nervous system and humoral 15 factors. Respiration gives rise to waves in heart rate mediated primarily via the parasympathetic nervous system, and it is thought that the lag in the baroceptor feedback loop may give rise to waves in heart rate. Factors that affect the input are the baroreflex, thermoregulation, hormones, sleep-wake cycle, meals, physical activity, and stress. iO Decreased parasympathetic nervous system activity or increased sympathetic nervous system activity will result in reduced heart rate variability. High frequency (HF) activity (0.15 to 0.40 Hz), especially, has been linked to parasympathetic nervous system activity. Activity in this range is associated with the respiratory sinus arrhythmia, a vagally mediated modulation of heart rate such that it increases during inspiration and decreases during i5 expiration. While the term heart rate variability can be simply explained in terms of the temporal variation between consecutive heartbeats, there are many methods for calculating and expressing this parameter. 'O On a standard electrocardiogram (ECG), the maximum upwards deflection of a normal QRS complex is at the peak of the R wave, and the duration between two adjacent R wave peaks is termed the R-R interval. The ECG signal requires editing before heart rate variability analysis can be performed, a process requiring the removal of all non-sinus-node-originating '5 beats. The resulting period between adjacent QRS complexes resulting from sinus node depolarizations is termed the N-N (normal-normal) interval. Heart rate variability is the measurement of the variability of the N-N intervals. Heart rate variability can be measured in time or frequency domains. Time domain methods 80 are the simplest to perform. Each N (or R) point is determined in the ECG trace and variables such as mean HR and longest and shortest N-N intervals calculated. More complex calculations such as SDNN (standard deviation of the N-N intervals, representing the overall heart rate variability) and NN50 (the number of adjacent N-N intervals that differ by more than 50 ms) can be performed using this data. Variables can also be derived that 85 estimate the short- and long-term components of heart rate variability (i.e. RMSDD, the square root of the mean squared differences between adjacent N-N intervals gives an estimate of short-term heart rate variability, and SDANN, the standard deviation of the average N-N interval over periods of about 5 min, gives an estimate of long-term heart rate variability). The calculation of all these variables enables the temporal variability of the HR to -8- )0 be quantified. The contribution of the various factors that manifest themselves in HF and LF HR changes can also be quantified (i.e. para-sympathetic and sympathetic influences). Spectral methods may be used to analyse heart rate variability. These measure how the variance (or power) of the ECG signal changes as a function of frequency. Non-parametric )5 methods of spectral analysis employing the Fast Fourier Transform (FFT) algorithm are commonly used. This technique involves splitting the ECG waveform into small subunits (usually from 2 to 5 min long for the measurement of HFP, LFP and VLFP, but can be up to 24 h when analysing ULF components). These signal segments are then 'transformed' from a temporal signal into a spectral representation whereby the ECG signal is reinterpreted as )0 the sum of multiple simpler (sinusoidal) waves of a given amplitude and frequency. The amplitudes of the component waves are then plotted to give a power spectrum by plotting power (the square of amplitude in volts) versus frequency. More recently, time-frequency signal analysis methods have been used in the analysis of )5 heart rate variability. As their name suggests, these offer simultaneous interpretation of the signal in both time and frequency, which allows local, transient or intermittent components to be elucidated. (These are often obscured due to the averaging inherent within spectral-only methods, i.e. the FFT.) Several time-frequency methods are currently available, including the short time Fourier transform (STFT), Wigner-Ville transform (WVT), Choi-Williams .0 distribution (CWD) and the continuous wavelet transform (CWT). Of these, the CWT has become the most favoured tool by researchers, as it does not contain the cross-terms inherent in the WVT and CWD methods, and provides frequency-dependent windowing, which allows for arbitrarily high resolution of the high frequency signal components (unlike the STFT). Accordingly, high frequency components (the 'fine detail' of the ECG signal) are 15 not lost to analysis. ECG-based methods are considered superior because they provide a clear waveform, which simplifies the exclusion heartbeats not originating in the SA node. 20 While possibly less precise, methods not reliant on ECG measurements will also find use in the present methods. For example, time-based blood pressure recordings, ballistocardiograms, and the pulse wave signal derived from a photoplethysmograph (PPG) are applicable. 25 Heart rate information useful in calculating heart rate variability may be obtained from consumer level devices (such as those incorporated into wrist or chest word devices), and -9also smart phones which utilise the on-board camera). Indeed, the prior art provides application software for use in smartphones capable of measuring heart rate and transforming the data into heart rate variability. io The present methods may be executed over a single IHT or IHHT treatment session (including even a partial session) or across multiple sessions. For example, a single session may comprise a 45 minute period whereby hypoxic and normoxic air is alternately breathed by the subject. Alteration of the oxygen concentration in the hypoxic air according to the 15 measured heart rate variability air occurs within the single session. Thus, where the hypoxic air has an oxygen concentration of 15%, and the measured heart rate variability is low then the oxygen concentration may be further reduced before the end of the session in order to stimulate a higher heart rate variability. o The present methods may be executed over multiple IHT or IHHT treatment sessions. For example, a treatment program may comprise ten single sessions of 45 minutes period whereby in each single session hypoxic and normoxic air is alternately breathed by the subject. Alteration of the oxygen concentration in the hypoxic air according to the measured heart rate variability occurs between the single sessions. Thus, where the hypoxic air has an 5 oxygen concentration of 15%, and the heart rate variability measured in the first session is low then for the second treatment session (which may occur days later) the oxygen concentration may be further reduced in order to stimulate a higher heart rate variability. In some embodiments of the method the oxygen concentration of the hypoxic air may be i0 altered both within a single session, and also altered across multiple sessions. For example, where oxygen concentration is reduced during a first session with little effect on heart rate variability, the second session may commence with any even lower oxygen concentration with that aim of triggering a significant variation in heart rate variability over the second session. 55 It is preferable for heart rate variability to be measured at multiple time points throughout a treatment session. For example, a heart rate variability measurement may be taken at the transition between each hypoxic and normoxic exposure period in order to determine whether the hypoxia is sufficient to improve heart rate variability. Better information may be 60 provided where heart rate variability is measured at regular, short time intervals (such as every minute, every 10 seconds, every second, or every tenth of a second). Alternatively, substantially continuous measurements may be taken. -10- In some embodiments, the oxygen concentration in the hypoxic air is altered in response to i5 every heart rate variability measurement. In this way, the composition of the air breathed by the subject is virtually instantaneously altered in response to an alteration (or a lack of alteration) in heart rate variability. For, example the subject's heart rate variability may be continuously measured, and the oxygen concentration in the hypoxic air continuously altered such that the oxygen level in the hypoxic air is decreased until an improvement in heart rate 'O variability results. When a desired improvement is detected, the level of oxygen may instantaneously stabilise. The improvement in heart rate variability may be an increase or decrease in heart rate variability, as required according to the clinical or performance endpoint required. '5 While it is generally considered that reduced heart rate variability is associated with lower survival or poorer prognosis of disease, this association may be reversed for healthy and/or high-performing subjects. For these subjects, increased heart rate variability may be associated with and/or predictive of increased stress levels and, therefore, poorer !0 performance, while reduced heart rate variability may be associated with lower stress and improved performance. For example, higher athletic performance may be associated with reduced heart rate variability. Accordingly, monitoring heart rate variability after activities that are considered stressful and/or taxing for a typical subject may allow for the identification of individuals who may be exceptional or high-performing (i.e., those with 15 reduced heart rate variability) as well as individuals who may be likely to fail a physical test (i.e., those with increased heart rate variability). In addition or alternatively to lowering the oxygen concentration, the length of a hypoxia time period within the session may be lengthened until a required improvement in heart rate variability is achieved. 90 In addition to, or alternatively to the altering of oxygen level or altering the hypoxia time period, an additional hypoxia time period may be added. In addition to, or alternatively to the altering the oxygen level or altering the hypoxia time period, or adding a further hypoxia period, the period of normoxia or hyperoxia may be 95 altered. . In some embodiments, the method may comprise at least one time period during which hyperoxic air is administered to the subject. In these embodiments, the heart rate variability may be measured such that the oxygen concentration in the hyperoxic air is altered so as to cause a required alteration to heart rate variability. 00 -11- In addition or alternatively to altering the oxygen concentration in the hyperoxic air, the length of a hyperoxia time period within the session may be lengthened until a required improvement in heart rate variability is achieved. )5 In addition to, or alternatively to the altering the oxygen level or altering the hyperxia time period, an additional hyperxia time period may be added. In a second aspect, the present invention provides a system for treating a subject using intermittent hypoxia, the system comprising: .0 a gas delivery means configured to provide a hypoxic air, and a heart rate variability measuring means, an automated control means configured to control the oxygen concentration in the hypoxic air and/or the time period for which a subject is exposed to the hypoxic air according to the measured heart rate variability. .5 The present system may be embodied in the form of a standalone collection of components arranged in the form of a workstation unit or similar. Components of the system may be disposed on a portable trolley, for example, to allow movement from room to room. 0 The gas delivery means may comprise a conduit (such as a pipe or flexible tube) for conveying a hypoxic gas (and optionally hyperoxic gas) to the subject. For the normoxic gas, it is contemplated that the gas delivery means may simply cease conveying gas to the subject and allow the subject to breathe atmospheric oxygen. 25 Alternatively, the gas delivery means may pump a normoxic gas via the conduit used to deliver the hypoxic gas to the subject. The gas delivery means may further comprise one more gas storage means (such as a tank) with the storage means containing a gas having a certain (hypoxic, or optionally 30 hyperoxic) concentration of oxygen. In some embodiments, two tanks may be used with a first tank having a gas substantially devoid of oxygen and a second tank having substantially pure oxygen with a valve-based mixing means configured to mix the tow gases so as to provide a gas mixture having the required gas concentration. The controller of the system may automatically adjust the gas mix to provide the required concentration of oxygen in the 35 gas breathed by the subject. -12- For example, streams of oxygen and nitrogen gas derived from separate gas tanks may be mixed in varying proportions by one of more electronically operated valves to achieve a desired percentage of oxygen. Exemplary systems have been previously described: Pedlar [0 C R, Howatson G, Whyte G P, Godfrey R J, Macutkiewicz D. Simulating moderate altitude using normobaric hypoxia with commercially available hypoxic gas generators. High Alt Med Biol. 2005 Winter;6(4):346-7; and also Artino A R, Folga R V, Swan B D, Mask-On Hypoxia Training for Tactical Jet Aviators: Evaluation of an Alternate Instructional Paradigm, Aviation, Space, and Environmental Medicine Vol. 77, No. 8 August 2006. [5 As an alternative to a tank-based supply, the gas delivery means may comprise a gas generating means capable of generating an oxygen depleted gas. Such devices are generally known in the art as hypoxicators. The skilled person is familiar with filtering technology used to alter the concentration of oxygen in air. One example is by a io semipermeable membrane that separates oxygen from nitrogen. Exemplary systems utilise semipermeable hollow fibre air separation methodologies. Preferably the gas generating means is controllable by the controller of the present system such that the amount of oxygen in the generated gas is set or alterable automatically. A 15 number of computer controlled hypoxicators are known in the art including those manufactured by Biomedtech Australia Pty Ltd. For example, the GO2Altitude hypoxic and hyperoxic air generator may be used in conjunction with the GO2Altitude hypoxicator station to provide accurately controlled levels of oxygen. The hypoxicator station is equipped with an interface allowing connection to a controller PC. iO Preferably the present methods and systems are utilized in a normobaric environment, and are preferred for reasons of simplicity and economy. However it will be understood that the present systems and methods may in some embodiments extend to hypobaric chambers to induce a hypoxic state in a subject. In such embodiments, the hypobaric chamber is 65 considered the gas delivery means. The heart rate variability measuring means comprises (i) means for detecting a heart beat and (ii) means for calculating the heart rate variability. The skilled person is familiar with many devices useful for detecting a heart beat, such devices generally used to measure 70 heart rate. Devices having an electrode configured to detect an electrical activity associated with a heart beat, or a microphone configured to detect a sound wave associated with a heart beat, or a pressure detector configured to detect a blood pressure change associated -13with a heart beat, or a camera configured to detect a visual change associated with a heart beat. '5 In one embodiment, the heart beat detecting means is controllable by the controller such that a measurement is taken only at a required time, and/or communicated for further analysis at a required time. Where the heart beat detecting means is capable of operating in different modes, the switching of modes may be achievable by a separate controller, such as the !0 controller of the present system. The means for calculating the heart rate variability is typically a processor-based device (such as a personal computer or a smartphone) capable of accepting the output (and preferably the digital output) of the means for identifying a heart beat. The calculating 15 means may be programmed to calculate heart rate variability according to any suitable method (including any of those disclosed herein in relation to the inventive methods). The output of the means for measuring heart rate variability will typically be digital for ease of incorporation into an entirely digital system. As described infra, the present system may )0 comprise algorithmic means for which the means for measuring heart rate variability provides an input parameter. The automated control means may be any mechanical and/or electrical and/or electronic device capable of controlling the oxygen concentration in the hypoxic air and/or the time )5 period for which a subject is exposed to the hypoxic air according to the measured heart rate variability. The automated control means, may be intrinsically capable of receiving as input the measured heart rate variability and setting or altering the oxygen concentration of the gas delivered by the gas delivery system. Typically, the intrinsic capability is provided by an integral processor. In addition or alternatively, the automated control means is controlled by 00 external means (such as a computer). As an example of a system whereby the controller comprises processor, but the controller is further controlled by an external computer, the gas delivery system may be a processor based hypoxicator which produces oxygen depleted gas by way of membrane separation 05 technology. The hypoxicator controls the gas generation processes by way of an on board processor, however is itself interfaced to a personal computer which instructs the hypoxicator to generate a gas of the required oxygen concentration. -14- Advantageously, the system is provided with a rule (or a set of rules) configured so as to .0 accept as input a heart rate variability value and provide as output oxygen concentration in the hypoxic air and/or the time period for which a subject is exposed to the hypoxic air. The oxygen concentration and/or time period is calculated according to the input heart rate variability. Conveniently, the rule(s) is/are embodied in electronic algorithmic form. .5 The algorithm provides the link in the biofeedback loop between the subject and the gas generating means, using heart rate variability or the alteration in heart rate variability, or the lack of alteration in hear rate variability as the feedback parameter. For example, a rule may dictate that in a single treatment session the oxygen concentration in the gas from the gas delivered by the gas delivery means decreases by 1% for each hypoxia period until an 10 increase in heart rate variability of 5% is noted. Once that level of heart rate variability is reached, the oxygen concentration level is kept fixed until the end of the session, unless heart rate variability returns to the base line level, in which case the oxygen concentration in the gas is further lowered until the 5% increase in heart rate variability is again noted. In this way, the subject is constantly challenged positively (with respect to the required heart rate .5 variability level) by virtue of the heart rate variability feedback loop of the system. The above algorithms are considered an "intra-session" algorithm because they rely on inputs from a current treatment session. !0 A subject who requires multiple hypoxia challenges at decreasing oxygen concentrations to maintain a given level of heart rate variability within a single treatment session could be considered as refractory. Accordingly, in the next treatment session better effect may be gained by avoiding mildly hypoxic gasses and to instead commence the session at a lower oxygen concentration than used in the previous session. 35 The algorithm may comprise a fixed set or rules, or may be able to "learn" (in an artificially intelligent manner) from past data sets. It is further contemplated that data used by the present method or systems or generated by 40 the present systems or methods may be uploaded to or downloaded from the internet. Thus, heart rate variability data may be evaluated by an algorithm on a server remote to the subject. The upload may be effected by a computer, or even a smartphone. With reference to the computer-related aspects of the invention the term "processor" may 45 refer to any device or portion of a device that processes electronic data, e.g., from registers -15and/or memory to transform that electronic data into other electronic data that, e.g., may be stored in registers and/or memory. A "computer" may include one or more processors. The systems and methodologies described herein are, in one embodiment, performable by i0 one or more processors that accept computer-readable (also called machine-readable) code containing a set of instructions that when executed by one or more of the processors carry out at least one of the methods described herein. Any processor capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken are included. Thus, one example is a typical processing system that includes one or more processors. Each 15 processor may include one or more of a CPU, a graphics processing unit, and a programmable DSP unit. The processing system further may include a memory subsystem including main RAM and/or a static RAM, and/or ROM. A bus subsystem may be included for communicating between the components. i0 The processing system further may be a distributed processing system with processors coupled by a network and could be a virtual processing system or a cloud based processing system. If the processing system requires a display, such a display may be included, e.g., a liquid i5 crystal display (LCD) or a light emitting diode (LED) display. If manual data entry is required, the processing system also includes an input device such as one or more of an alphanumeric input unit such as a keyboard, a pointing control device such as a mouse, and so forth. 70 The term memory unit as used herein, if clear from the context and unless explicitly stated otherwise, also encompasses a storage system such as a disk drive unit. The processing system in some configurations may include a sound output device, and a network interface device. The memory subsystem thus includes a computer-readable carrier medium that carries computer-readable code (e.g., software) including a set of instructions to cause 75 performing, when executed by one or more processors, one of more of the methods described herein. Note that when the method includes several elements, e.g., several steps, no ordering of such elements is implied, unless specifically stated. The software may reside in the hard disk, hard drive, memory stick, flash memory card or like device, or may also reside, completely or at least partially, within the RAM and/or within the processor during 80 execution thereof by the computer system. Thus, the memory and the, processor also constitute computer-readable carrier medium carrying computer-readable code. -16- Furthermore, a computer-readable carrier medium may form, or be included in a computer program product. 15 Note that while descriptions and diagrams may only refer to a single processor and a single memory that carries the computer-readable code, those in the art will understand that many of the components described above are included, but not explicitly shown or described in order not to obscure the inventive aspect. )0 The present systems may comprise a computer-readable carrier medium carrying a set of instructions, e.g., a computer program that is for execution on one or more processors, e.g., one or more processors. Thus, as will be appreciated by those skilled in the art, embodiments of the present invention may be embodied as a method, an apparatus such as )5 a special purpose apparatus, an apparatus such as a data processing system, or a computer-readable carrier medium, e.g., a computer program product. The computer readable carrier medium carries computer readable code including a set of instructions that when executed on one or more processors cause the processor or processors to implement a method. Accordingly, aspects of the present invention may take the form of a method, an )0 entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present invention may take the form of carrier medium (e.g., a computer program product on a computer-readable storage medium) carrying computer-readable program code embodied in the medium. )5 The software may further be transmitted or received over a network via a network interface device. While the carrier medium is shown in an exemplary embodiment to be a single medium, the term "carrier medium" should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term "carrier medium" shall also be taken 10 to include any medium that is capable of storing, encoding or carrying a set of instructions for execution by one or more of the processors and that cause the one or more processors to perform any one or more of the methodologies of the present invention. A carrier medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. 15 Non-volatile media includes, for example, optical, magnetic disks, magneto-optical disks, flash drives, and the like. Volatile media includes dynamic memory, such as main memory. Transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise a bus subsystem. -17- -0 Transmission media also may also take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. For example, the term "carrier medium" shall accordingly be taken to included, but not be limited to, solid-state memories, a computer product embodied in optical and magnetic media; a medium bearing 5 a propagated signal detectable by at least one processor of one or more processors and representing a set of instructions that, when executed, implement a method; and a transmission medium in a network bearing a propagated signal detectable by at least one processor of the one or more processors and representing the set of instructions. !0 It will be understood that the steps of methods discussed are performed in one embodiment by an appropriate processor (or processors) of a processing (i.e., computer) system executing instructions (computer-readable code) stored in storage. It will also be understood that the invention is not limited to any particular implementation or programming technique and that the invention may be implemented using any appropriate techniques for 15 implementing the functionality described herein. The invention is not limited to any particular programming language or operating system. It should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single [0 embodiment, figure, or description thereof, for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing 45 disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention. Furthermore, while some embodiments described herein include some but not other features 50 included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments may be used in any combination. 55 In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific -18details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description. iO Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. i5 Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention. The present invention will now be more fully described by reference to the following non 'O limiting embodiments. PREFERRED EMBODIMENT OF THE INVENTION Reference is made to Fig. 1. A pulse oximeter 10 that includes optical components such as '5 a light emitter (e.g., a light emitting diode) and a light detector (e.g., a photodetector) is applied to a subject 12 and receives sensed light 14 which is used by the oximeter 10 to generate a plethysmographic waveform. The oximeter 10 is coupled to a personal computer 16 appropriate for heart rate variability monitoring, the coupling being via the internal computer bus 18. The oximeter 10 is controlled by driver softer 20. The driver software 20 !0 also receives data 22 from the oximeter 10 and communicates the data to the computer CPU 24 for transformation. A time processing unit (TPU) provides timing control signals to drive circuitry controlling 22 when the pulse oximeter 10 sensor is activated. TPU also controls the gating-in of signals 85 from the pulse oximeter 10 through a switching circuit. These signals are sampled at the proper time, depending at least in part upon which of multiple light sources is activated, if multiple light sources are used. The received signal from the pulse oximetry sensor is passed through an amplifier, a low pass filter, and an analog-to-digital converter. The digital data is then stored in a queued serial module (QSM), for later processing. 90 Based at least in part upon the received signals corresponding to the light received by optical components of the pulse oximeter 10 sensor, CPU 24 calculates a heart rate variability using an algorithm 34 stored in the personal computer 16. The calculation method -19may employ certain coefficients, which may be empirically determined, and may correspond )5 to the wavelengths of light used. The calculation method and coefficients are stored in the computer 26 memory and accessed and operated according to CPU 24 instructions. In one embodiment, the correction coefficients are provided as a lookup table. Heart rate variability may be determined from a plethysmographic waveform with relatively )0 high resolution. A high resolution signal may capture smaller differences in beat-to-beat heart rate variability that may be masked by a lower resolution signal. The processor determines the patient's physiological characteristics, such as Sp02 and pulse rate, using various algorithms 34 and/or look-up tables based generally on the value of the received signals corresponding to the light received by the detector. )5 Higher resolution signals may be obtained via continuous wavelet transformation as disclosed in U.S. application Ser. No. 12/437,317, entitled "Concatenated Scalograms," filed May 7, 2009, and incorporated herein by reference in its entirety for all purposes. Embodiments of the present disclosure may utilize systems and methods such as those .0 disclosed in U.S. application Ser. No. 12/437, 317, for obtaining information from the received signal to determine and to detect changes in physiological conditions. The sampling rate for sampling of the analog signal by the analog-to-digital converter is at least 2000 Hz. .5 Certain types of signal processing may influence the ability of the CPU 24 to detect rapid changes in heart rate. Accordingly the pulse oximetry signal from the oximeter 10 may be passed to the CPU 24 before any filtering has occurred (or may be passed to the microprocessor after only minimal filtering, such as low pass filtering). The signal may be 20 used to calculate the heart rate variability, while additional processing and filtering (e.g., amplifier, and a low pass filter) may be applied the signal. The signal may be sampled at a rate appropriate for calculating heart rate variability by a second analog-to-digital converter. Such a minimized signal processing arrangement prior to application of a heart rate variability calculation may facilitate detection of more rapid changes in beat-to-beat variation 25 relative to a heavily processed signal. The pulse interval validity is be determined by a rules-based method for determining allowable variation from historical and/or a calculated mean of a particular patient and by considering criteria such as artefact rejection, waveform smoothness, and noise between 30 pulses. If the pulse interval is invalid, the method waits for additional data. If the method -20determines that a suitable pulse interval has been collected over a suitable period of time, then a calculation of the heart rate variability is performed. To the personal computer 16 there is also connected a computer controllable hypoxicator 26 15 (go2altitude@ OnePlus R3 (Biomedtech Australia Pty Ltd) allowing data transmission 32. The hypoxicator 26 uses membrane separation technology to provide hypoxic and hyperoxic air 28 to the subject 12 via a face mask. The personal computer 16 comprises software 30 (adapted from go2altitude@ PC software; [ Biomedtech Australia Pty Ltd) configured to act a driver and also to instruct the hypoxicator 26 to generate air of the required oxygen concentration. The personal computer further comprises an algorithm 34 executable by the CPU 24. The algorithm receives (via CPU 24) data input from the oximeter 10 and transforms the oximeter 5 data into instructions (according to the rules of the algorithm 34), the instructions communicated 32 to the hypoxicator 28. In response to the instructions, the hypoxicator 26 alters the oxygen concentration of the air administered to the user 12. Thus, it may be seen from the above that a closed feedback loop is described by the arrows 0 22, 32, 28 and 14 whereby the subject's 12 heart rate variability directs the oxygen concentration of air generated by the hypoxicator 26. The air is administered 28 to the subject 12, with inhalation of that air potentially altering the heart rate variability of the subject 12. 55 The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein 60 represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art. 65 -21-