Why Heart Rate Variability (HRV) is a Key Parameter in Rewire’s Readiness Score

If you are an athlete (of any level) you have most likely tracked your workouts in a number of ways. From subjective metrics such as the rate of perceived exertion (RPE, or how hard a workout feels), to distance, duration, power, heart rate, pace or speed, as well as compound metrics such as Strava’s Relative Effort or Training Peak’s TSS, these can all serve a purpose in our quest to quantify the stimulus we apply with training. 

However, an equally important (or maybe more important?) question we want to answer is the following: how are you responding to training? After you went out for your session, did your body bounce back from that homeostatic disruption? How long did it take? Are you ready for another high intensity session or should you take it easy another day or two?

Being able to answer these questions can help us avoid a potential state of negative adaptation and hinder performance outcomes in the long term. Here is when Heart Rate Variability (HRV) comes to the rescue. 

What is HRV?

HRV is a term that refers to ways to summarize in a number the variability between heartbeats. The variation between heartbeats results from the activity of the autonomic nervous system in response to stress. As the body is continuously re-adjusting to maintain a state of balance, called homeostasis, heart rate, blood pressure, glucose level, hormones, etc. — react to the challenges we face and the autonomic nervous system works to keep everything in balance so that we can function optimally (e.g. do not develop chronic conditions, or improve our performance). Heart rhythm (and therefore HRV) is regulated by the parasympathetic branch of the autonomic nervous system, the one in charge of rest and relaxation. Hence, measuring HRV is an effective way to capture how the body is doing while trying to maintain a state of balance in response to different stressors (training, lifestyle, etc.).

Example of a few seconds of ECG data, including detected beats. The time differences between beats are called RR intervals and are the basic unit of information used to compute HRV. We need several RR intervals to be able to compute your HRV. This is why HRV needs to be computed over a certain amount of time, typically between 1 and 5 minutes.

​In particular, a reduction in certain HRV features typically means that parasympathetic activity is reduced, and therefore we have not fully recovered or in general, there is more stress in our lives. At rest, the body is predominantly parasympathetic, which is why HRV analysis today is mostly focused on identifying reductions in parasympathetic activity, captured by features such as rMSSD (the root mean square of successive differences in RR intervals). The use of rMSSD is motivated by physiological mechanisms: the vagus nerve acts on receptors signaling nodes to modulate pulse on a beat to beat basis while sympathetic activity has different pathways with slower signaling. Hence beat to beat changes captured mathematically by rMSSD reflect parasympathetic activity, also called vagal influence. 

This means that when we train or face other stressors, HRV is typically reduced at the acute level (during and right after the stressor). Additionally, if the stressor is particularly large (say, a hard race), or if we are responding poorly to a series of stressors (for example a block of high intensity sessions), HRV can remain suppressed for several days or longer. This is a typical sign of negative adaptation, something we can avoid by better managing and adjustring training, based on our unique physiological response. 

What about non-training related stressors?

One of the key aspects of measuring HRV and using it to gauge readiness or guide training, is its ability to track your stress response regardless of the source. What does this mean? No matter if stress comes from training, work, getting sick, poor lifestyle, or some unexpected event, it will have an effect on our ability to cope with additional stressors and perform. HRV is an overall marker of stress and will be affected by pretty much any factor that has an influence on your autonomic nervous system, making it a great tool for training management.

How can you measure your HRV with Rewire?

HRV forms a part of Rewire’s Readiness Assessment and can be measured live in the background from your Bluetooth Heart Rate Monitor. It can also be read from a health app like Oura or Apple Health. We have seen how HRV is a global marker of stress and also how it is typically impacted acutely by any sort of stressor. This comes at a cost: we cannot just measure HRV anytime and use the data reliably, as HRV will typically reflect changes in heart rate modulation due to a myriad of transitory stressors we might not be really interested in (e.g. having coffee, or walking up the stairs). Timing of the measurement becomes key if we want to assess baseline physiological stress in response to larger acute and chronic stressors, and use this data for daily adjustments. 

Measurement time

Almost the entirety of research up to date has been carried out with morning HRV measurements, hence this is typically the preferred protocol and also what is implemented in Rewire, where you can also set reminders to help you make the morning readiness assessment a part of your daily morning routine

The Readiness Assessment should be taken first thing in the morning, while in a rested physiological state. While in the past subjects in clinical studies were asked to go to the lab, avoiding eating, drinking and exercising in the 2 hours preceding a measurement, waiting between 10 and 30 minutes before the measurement to get back into that relaxed state, things are much simpler now due to the technological improvements that allow users to measure simply using their phones. Ideally, measurements should be taken as soon as a person wakes up, while still in bed. The morning routine, or having a standard measurement protocol should sound familiar in many situations, for example measuring weight before breakfast, measuring blood pressure in standard conditions (sitting, arm position, etc), and similarly, assessing readiness to determine the impact of training and lifestyle on physiological stress and recovery needs.

Body position and measurement duration

In terms of body position, lying down, sitting or standing are good alternatives, but in case you do not lie down, make sure to wait a few seconds before measuring, and use the same body position each day. Several studies have also shown that for time domain features representative of parasympathetic activity, such as rMSSD, the most commonly used metric in today’s tools, 60 seconds are sufficient. 

Measurement Frequency

Measuring Readiness daily is best to obtain useful data as it establishes a strong baseline for HRV. It also means you can check in regularly with your readiness, allowing you to make smarter training and recovery decisions. Since Rewire also collects a range of cumulative data points such as training load and mental load, checking in daily ensures that there is a more complete data set involving the highs and lows of your training and work. Measuring daily is also often easier to remember, since it can form a part of your regular morning routine.

What to do (and not to do) while measuring 

During the assessment, movement should be avoided, but there are also other aspects that can trigger artifacts and require a little more attention. In particular, yawning and swallowing should also be avoided, the latter for example causes a sort of instantaneous bradycardia that can affect the measurement. 


HRV is affected by breathing. The question of using controlled or paced breathing or breathing naturally needs to be analyzed in the context of our target application, which is measuring physiological (chronic) stress first thing in the morning, longitudinally within an individual. One of the main reasons behind using paced breathing is that it is supposed to make the measurement more reliable and improve measurement repeatability. In our experience, this is not the case and letting people breathe freely feels much easier to most. In our tests we have highlighted how self-paced and paced breathing result in the same differences between consecutive measurements, hence proving that one way or the other is as effective. Thus, Rewire does not use paced breathing as part of the readiness assessment. 

How can you use the data to adjust training?

At the beginning of this blog, I covered the physiological underpinnings of HRV measurement as well as key aspects of data collection: context and best practices. By following best practices meaningful data points truly representative of physiological stress can be collected. As technology today allows for easy data collection, many of the basic physiological mechanisms behind applied use of HRV (for example the acute drop in HRV after hard workouts) have been successfully identified in user-generated data. These types of analysis provide further evidence of the effectiveness of today’s technologies in capturing individual responses to stress. ​It’s important to remember that physiology is complex, and while acute stressors (such as a hard workout) and the resulting HRV changes are often repeatable and easy to understand, there might be other factors behind the relationships that we are seeing (or not seeing) in our data. No stressor acts in isolation, there’s always something going on with our lifestyle, training, health, and so on. 

Let’s look at how we can use the data to adjust training. In the past decade, we have seen how HRV has been used to capture changes in training load, fitness and performance. In a landmark study, Kiviniemi et al. proposed a first protocol to guide training based on HRV readings, and analyzed changes in training load and VO2max in recreational runners following an HRV-guided program, compared to controls following regular periodization. The authors state that the basic idea of HRV guided training was to decrease the training stimulus when HRV decreased and maintain training stimulus high when HRV remained the same or increased. Often, HRV-guided training results in lower frequency of high intensity exercises compared to the control group. This is a common theme as most protocols aim at avoiding the application of too strong a stressor (e.g. a hard session) when the athlete is not physiologically ready (e.g. when HRV shows high stress present on the body). Based on this data, HRV guided training may adjust both the timing and amount of high-intensity exercises at individual level. Yet, in these studies, performance for the HRV-guided group improved, showing how the timing of the high intensity sessions does matter. Rewire uses HRV as well as other objective and subjective measures to provide you with training and recovery guidance. Readiness-guided training aims at providing the most appropriate training stimuli in a timely manner, when the body is ready to take it, so that positive adaptation will occur, leading to better health and performance outcomes.

After the initial studies by Kiviniemi et al. most researchers shifted their approach to one less coupled to day to day variability and acute stressors, trying to look at medium and long term trends and more significant stressors that might affect physiology chronically. With the new approach, we do not really care if a single daily score is below baseline, what we care about is that the baseline itself does not go below normal values. Intuitively, for the baseline to go below normal values, we need quite a few “bad days” (low HRV scores), therefore adjustring training less often and only when a stronger negative response is present.

​Vesterinen et al. were able to show improved performance for the HRV-guided group using this protocol. In particular, the number of high intensity workouts was lower for the HRV-guided group, but despite the lower amount of high intensity exercise, the group was able to improve running performance over a 3000 m time trial. In a similar study, Javaloyes et al. examined the effect of training prescription based on HRV in road cycling performance. After 4 weeks baseline measurements, 17 well-trained cyclists were split into two groups, HRV-guided and traditional periodisation group. The training program lasted another 8 weeks, and performance measures were taken before and after the 8 weeks in both groups. In the study, the HRV guided group improved peak power output (by 5%)  and 40 minutes time trial performance (by 7%), while the traditional periodisation group did not improve in any metric. The authors conclude that daily training prescription based on HRV could result in a better performance enhancement than a traditional periodization in well-trained cyclists.

Rewire‘s algorithm builds HRV into it’s readiness scores alongside a range of additional subjective and objective measures to provide a holistic approach to readiness. HRV impacts both Rewire’s Overall and Physical readiness score and training recommendations are provided accordingly. Rewire also provides a Personalized Recovery Session that isolates particular weak points in your state and selects a session tailored to your goals for that day. This combination of training and recovery recommendations allows you to perform at your best.

Wrap up

In this blog, we have covered the basics of HRV, and why it matters. We have also provided useful tips and best practices for your morning Readiness Assessment, so that you can collect high-quality data representative of changes in baseline physiological stress, using the Rewire app.

Needless to say, HRV is not the only relevant marker to quantify readiness, and should be integrated with information related to training load as well as subjective metrics such as stress, frustration or muscle soreness, all aspects that might independently indicate potential issues. Rewire offers an integrated approach to readiness that combines all of these parameters to provide you with a comprehensive view of your readiness to train.

The Top 5 Stoic Quotes You Need to Know to Build Mental Toughness

Stoicism is an ancient school of thought founded by Zeno of Citium. Stoic Philosophy teaches that the path to eudaimonia (or happiness) is found through controlling the controllable and accepting the uncontrollable as they happen; living with Areté (or excellence/moral virtue), and by taking responsibility. Try to use these stoic quotes as an anchor on your journey towards mental toughness.

You have power over your mind – not outside events. Realize this, and you will find strength.

Marcus Aurelius

How long are you going to wait before you demand the best for yourself?


A gem cannot be polished without friction, nor a man [or woman] perfected without trials.


Man [or woman] conquers the world by conquering themselves.

Zeno of Citium

Be tolerant with others and strict with yourself.

Marcus Aurelius

Write down these stoic quotes somewhere that you can see and keep reminding yourself of them as you work on developing your mental toughness.

A 101 Guide to Binaural Beats

Binaural beats are a form of brainwave entrainment that have been shown to have positive effects on stress, anxiety (1,2), focus (3), motivation, confidence and meditation (4). Binaural beats work when two different frequencies are heard, one in each ear. This creates a third tone, the binaural beat, whose frequency is the difference between the two other tones, e.g. if the tone in one ear is 400Hz and the other is 410Hz, the binaural beat is 10Hz. This binaural beat is shown to have a positive impact on the user’s mindset. It is important to note that stereo headphones are required to achieve a binaural beat since when using a speaker or non-stereo headphones the frequencies are already mixed outside the brain and hence no binaural beat is created.

The various tones of binaural beats affect the user differently. The following tones are used in the Rewire Mindset Recovery System:

  • 0.5 – 3.5 Hz – Delta wave for deep sleep
    • In a 2018 study, participants who received this frequency during sleep entered deep sleep quicker and for longer (5). This allows participants to gain more of the benefits of deep sleep including physical recovery.
  • 4.0 – 6.5 Hz – Theta for meditation/sleep
    • A 2017 study showed that even listening to a 6Hz binaural beat for just 10 minutes induced the user’s brain into a state similar to that achieved during meditation (4).
  • 7.0 – 12.5 Hz – Alpha for relaxation/dreams
    • In a 1990 study, alpha wave binaural beats were shown to have a positive influence on the user’s relaxation (6).
  • 13.0 – 38.5 – Beta for Activity            
    • It has been shown that beta wave binaural beats can positively affect vigilance performance and mood (3), and a recent 2019 study showed that beta wave binaural beats have a positive impact on long term memory (7).

Binaural beats also have a positive effect in counteracting the negative effects of mental fatigue. A recent 2020 study by Walter Staiano, Rewire’s Scientific Advisor, showed that binaural beats reduce the negative effect of mental fatigue (8). This makes binaural beats an important part of mindset recovery and pre-competition preparation in sports to minimise the negative effects that mental fatigue is shown to have on endurance performance (9,10).

We have put together a demo of our mindset recovery system featuring theta wave binaural beats which you can watch and download below.

You can also hear what binaural beats sound like raw and play around with different tones here.


1.        Padmanabhan R, Hildreth AJ, Laws D. A prospective, randomised, controlled study examining binaural beat audio and pre-operative anxiety in patients undergoing general anaesthesia for day case surgery. Anaesthesia. 2005; 

2.        Garcia-Argibay M, Santed MA, Reales JM. Efficacy of binaural auditory beats in cognition, anxiety, and pain perception: a meta-analysis. Psychol Res. 2019; 

3.        Lane JD, Kasian SJ, Owens JE, Marsh GR. Binaural auditory beats affect vigilance performance and mood. Physiol Behav. 1998; 

4.        Jirakittayakorn N, Wongsawat Y. Brain responses to a 6-Hz binaural beat: Effects on general theta rhythm and frontal midline theta activity. Front Neurosci. 2017; 

5.        Jirakittayakorn N, Wongsawat Y. A Novel Insight of Effects of a 3-Hz Binaural Beat on Sleep Stages During Sleep. Front Hum Neurosci. 2018; 

6.        Foster DS. EEG and Subjective Correlates of Alpha-Frequency Binaural-Beat Stimulation Combined with Alpha Biofeedback. 1990; 

7.        Garcia-Argibay M, Santed MA, Reales JM. Binaural auditory beats affect long-term memory. Psychol Res. 2019; 

8.        Axelsen JL, Kirk U, Staiano W. On-the-Spot Binaural Beats and Mindfulness Reduces the Effect of Mental Fatigue. J Cogn Enhanc. 2020; 

9.        Marcora SM, Staiano W, Manning V. Mental fatigue impairs physical performance in humans. J Appl Physiol. 2009; 

10.      Lopes TR, Oliveira DM, Simurro PB, Akiba HT, Nakamura FY, Okano AH, et al. No Sex Difference in Mental Fatigue Effect on High-Level Runners’ Aerobic Performance. Med Sci Sport Exerc. 2020;Volume Pub. 

An Athlete’s Guide to Looking After Yourself During​ Isolation

Across the globe, people are limited, like never before, with what they can and can’t do. This has, naturally, had a massive impact on everyone’s life and health, meaning that it is more essential than ever to make sure that we tick all the boxes in terms of our health.


During this time, it is incredibly important to keep exercising. Since (almost) all sporting competitions and events across the globe have been cancelled, it can become very demotivating to work without a goal. On top of this, with gyms closed and social distancing measures meaning that we cannot train with a partner, more barriers are in the way of our goals. The ability to overcome obstacles is what separates us as athletes.

“Obstacles don’t have to stop you. If you run into a wall, don’t turn around and give up. Figure out how to climb it, go through it, or work around it.”

Michael Jordan

At the very minimum, we want to maintain our fitness, however by using this unprecedented time to do more than anyone else, it distinguishes you and you will be ahead of the rest when we are released back into the sporting world again. One crucial thing to note is that it is important to do something even if it is significantly less than before. One study into resistance training showed that even when doing 1/9th of the original training, participants were able to maintain their muscle mass for the duration of a 32 week period, whereas those who did not undergo any training over 32 weeks regressed to their original muscle mass before the training plan began (1). However, now is not the time for complacency or downheartedness – we have a unique opportunity to use this time to progress and focus on our health in a way that we have never before.

It is also important to consider the positive effect that exercise has on our mental health. Exercise has been shown to reduce depression and anxiety and has positive effects on mood, self-esteem and cognitive function (2). This is another of the many reasons why we should continue to exercise regularly.


With significantly reduced social commitments it is very easy to let our sleep habits go to pot. Aside from having a negative effect on our physical and mental recovery, poor sleep quality impairs our immune system (3), something we naturally don’t want during a global pandemic. When sleeping, there are a few crucial things to take into account: consistency, appropriate length and quality – all which influence each other. By setting a consistent bedtime and wake up time you can reduce the time it takes to fall asleep as well as improving the quality of your sleep by allowing you to spend appropriate time in each sleep stage. When considering sleep length it is important to remember not only to get enough sleep but also not to get too much, having too little or too much sleep is associated with worse health (4). We cannot ‘store’ sleep and so once we have achieved all the sleep we need, sleep is essentially a waste of time and time that we can utilise for better purposes. When considering appropriate sleep length it can also be helpful to think in terms of 90-minute cycles, rather than hours – a more accurate representation of our sleep. You can use an app such as SleepCycle to track your sleep and wake you up at the lightest point so you feel free and ready to start the day. It is crucial to practice good sleep habits, try to turn off blue light sources as you prepare to sleep so as not to interfere with melatonin secretion, which is inhibited by the presence of blue light. To read more on sleeping efficiently, click here.


Strictly speaking, the immune system cannot be boosted through vitamins. However, vitamins can be used to support normal immune function. Whilst it might not be necessary to supplement vitamins or minerals, it is important to get a sufficient amount in your diet, and if you are unable to achieve suitable levels with your diet, then consider changing your diet or supplementing. Essential vitamins and minerals for supporting immune function are copper, folate, iron, selenium, zinc and vitamins A, B6, B12, C and D (5). You might find it useful to use an app, such as Cronometer, to track your micronutrient intake and highlight deficiencies. One vitamin to highlight in particular is vitamin D, which we get from sunlight. Considering the lockdown measures that many countries have put in this creates a challenge, particularly knowing that in normal circumstances 40% of Americans are deficient in vitamin D (6). If you have a garden, try to use it throughout the day or if your current government regulations allow, try to get outside, whilst abiding by social distancing measures. If you are unable to get outside, then consider supplementing vitamin D. Overall maintaining a healthy diet, rich in the relevant nutrients can help to maintain immune function during this crucial time.


During this strange period, it is easy for us to become stressed and distressed by the circumstances. To help combat this, set aside some time daily to practice mindfulness and breathing exercises. Mindfulness can help us to reduce stress (7), something that we all will be facing to varying extents with the forced change in lifestyle. At Rewire, we have been developing a Mindset Recovery System. This involves guided box breathing exercises (a Navy SEAL technique) to help reduce stress (8) and binaural beats which help to counteract the negative effects of mental fatigue (9).

We have put together a demo of these to help with your mindfulness during this difficult time, which you can access using the following video.

To read more about the positive effects of mindfulness, click here.

Overall, it is incredibly important to stay healthy and active during these uncertain times. Try to create structure in your life where structure lacks and use this unique time effectively to improve yourself without social pressure.


1.        Bickel CS, Cross JM, Bamman MM. Exercise dosing to retain resistance training adaptations in young and older adults. Medicine and Science in Sports and Exercise. 2011. 

2.        Callaghan P. Exercise: A neglected intervention in mental health care? Journal of Psychiatric and Mental Health Nursing. 2004. 

3.        Besedovsky L, Lange T, Haack M. The sleep-immune crosstalk in health and disease. Physiol Rev. 2019; 

4.        Kim CE, Shin S, Lee HW, Lim J, Lee JK, Shin A, et al. Association between sleep duration and metabolic syndrome: A cross-sectional study. BMC Public Health. 2018; 

5.        British Dietetic Association. COVID-19 / Coronavirus – Advice for the General Public. 2020. 

6.        Forrest KYZ, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011; 

7.        Astin JA. Stress reduction through mindfulness meditation. Psychother Psychosom. 1997; 

8.        Stinson A. What is box breathing? Medical News Today. 2018. 

9.        Axelsen JL, Kirk U, Staiano W. On-the-Spot Binaural Beats and Mindfulness Reduces the Effect of Mental Fatigue. J Cogn Enhanc. 2020; 

Simple Ways to Improve your Sleep for Athletic Performance

The quality of our sleep has huge implications for athletic performance. Sleep is essentially the time when physical and mental recovery occurs. With good cognitive function and physical readiness being required for us to perform at our peak, it is obvious to see how it is important that we have good quality sleep to perform at our best.

Start listening to our circadian rhythms

“We are the supremely arrogant species; we feel we can abandon four billion years of evolution and ignore the fact that we have evolved under a light-dark cycle. What we do as a species, perhaps uniquely, is override the clock. And long-term acting against the clock can lead to serious health problems”

Professor Russell Foster

Our sleep patterns our guided by circadian rhythms, which essentially act as our body clock and determine the appropriate time for hormone release, which in the case of sleep is melatonin. With the invention of the light bulb and screens has come the ability to overcome this natural body clock. The presence of blue light reduces the secretion of melatonin, increasing alertness and keeping us awake. This delays the onset of sleep and reduces the amount of time that we spend asleep. By cutting out screens as you prepare to go to sleep you can ensure that your onset of sleep is faster and thus your time in bed is more efficient.

Think in cycles not hours

“Eight hours sleep is an average amount of sleep people get per night, and it somehow seems to have become a recommended amount – for everyone. The resultant pressure put on getting this is incredibly damaging and counterproductive to getting the right amount of sleep that we individually need”

Nick Littlehales

To maximise the quality of our sleep we should quantify sleep in terms of cycles, not hours. One sleep cycle lasts approximately 90 minutes and hence if we can time our sleep to wake up at the end of a cycle we can wake up at the lightest point of our sleep and feel refreshed and ready to start the day.

We should also not be too concerned about the amount of sleep we get each day but instead over the whole week. In his book “Sleep”, Littlehales suggests that we should be getting 35 cycles per week, averaging 5 cycles a day, which works out as 7 hours 30 minutes. This approach is much more achievable than consistently hitting 8 hours which can be quite pressuring and stressful and a cycle approach is reflective of how we actually sleep. Littlehales says that this approach reduces the stress hormones released from struggling to sleep and allows us to get effective rest and recovery.

Apps like Sleep Cycle ensure that your alarm wakes you up at the lightest point of sleep possible meaning that you wake up feeling refreshed and ready to start the day.

Be more consistent

Our body adapts to the time we fall asleep and thus by being more consistent, we can fall asleep quicker and ensure the time that we spent in bed is efficient. Not only is our sleep more efficient when we are consistent, but we also get more slow-wave and REM sleep – the times when physical and mental recovery occurs respectively. This allows us to maximise the benefits of training and perform at our best. 

Laura Kline, Rewire Athlete, tells us that by developing a consistent routine she has been able to ensure she gets adequate sleep. “By 8:00 I have my magnesium drink and try to limit my screen time. I aim to be in bed by 9:30 – I find that following a set schedule makes a difference as my body knows it’s time to shut down.” Laura says that by doing this she can typically fall asleep within minutes and on the inevitable days that she can’t follow her routine she notices a difference the next day. 

Optimising our sleep is not necessarily about getting more sleep, but about making the time that we spend in bed as efficient as possible. Only by working to improve the way that we sleep can we truly allow for sufficient recovery to develop our athletic performance.

Further Reading:

Blue light from light-emitting diodes elicits a dose dependent suppression of melatonin in humans
West et al.
Journal of Applied Physiology, 2011

Sleep: Change the way you sleep with this 90 minute read
Nick Littlehales

New Feature: Sleep Consistency – Why We Track it, How Do You Compare?

‘Arrogance’ of ignoring need for sleep
James Gallagher