Your brain is one of the most important organs in the body. Understanding your health and wellness requires having a good sense of what is going on in the brain. However, most of our sensors are not in the brain. The Apple Watch is on your wrist, far from your brain. Most people are only monitoring their steps, which tells us little about brain function.

How can we know what is happening and provide accurate monitoring of your health? Part of the answer comes from a new metric called Heart Rate Variability, which in the last few years has become possible to easily and passively track.

This is part of the secret sauce that allows the Health Score to do fantastic things like warn you when your immune system is weaker, or give a detailed score of your sleep quality when you wake up in the morning.

What’s HRV?

Heart Rate Variability (HRV) is a measure of the variations in time (milliseconds) in between each heartbeat. The variations are a result of minute changes in your Autonomic Nervous System (ANS), and provide valuable insights into various areas of your health.

For something as important as your heart, you may think that variability is a bad thing. However, when you are at rest, this variability is actually quite desirable and a signal that your body is not in a fight-or-flight state (sympathetic activation of ANS).

What causes variability?

The ANS is made up of two halves — the sympathetic and parasympathetic nervous systems. You can think of them as the gas pedal and brakes in a car, or start and stop. Both need to function together in order to find balance and navigate through life.

The sympathetic nervous system is your body’s gas pedal, responsible for what is commonly called the fight or flight (or freeze) response. This response kicks in during moments of stress or excitement, and triggers various parts of your body to react, including raising your heart rate and pumping blood to your muscles to prepare for whatever is about to happen.

The parasympathetic nervous system is responsible for the relaxation response, sometimes called rest & digest. This helps you hit the brakes so your body can slow down, conserve energy, and perform maintenance tasks like digesting your food. Neither side is good or bad. Just like a car needs both brakes and gas to properly operate, you need both halves of your nervous system to function at your best!

When you’re parking your car, you want to be able to use more braking action without flooring the gas. Similarly, when you’re asleep or resting, you don’t want your sympathetic nervous system revving up a stress response or preparing to fight. On the other hand, if you are driving on the freeway you would want to use the engine and not just slam on the brakes. Similarly, when you are in an intense workout or trying to solve a hard problem, you don’t want a low, relaxed heart rate and may need the sympathetic nervous system to be more activated.

HRV is powerful because it can show your status as it goes between the two systems. Gyroscope is not implanting electrodes deep into your brain to physically check your brain activity, but through very non-invasive and convenient methods on your wrist or finger, we can still understand this nervous system activity to a fairly high degree of accuracy. This is all possible through minute changes in your heart rate patterns!

Why it matters

These HRV readings, with enough context and an accurate baseline, can act as precursors to anxiety, stress, panic and even illness. Or they can be indicators of cardiovascular fitness, and provide useful personal feedback about your lifestyle.

If your system is in a sympathetic state ("fight-or-flight" mode), the variation between each heartbeat tends to be low and the HRV numbers will also be low. If you're in a more relaxed state, the variation in heartrate is higher. Research continues to show a relationship between low HRV and mental health issues, as well as an increased risk of death and cardiovascular disease.

Many people focus on bodyfat percentage as one of the main metrics to track improvements in health, but HRV may be an even more useful north-star metric to keep an eye on once you have reached your body composition goals. It encapsulates all aspects of your well-being, and shows you how your own body is assessing its current risks and stress levels — including from high bodyfat levels, hormone levels, physical and mental stressors, inflammation, sleep debt, and many other signals.

How do I check my HRV?

The best way to track your HRV is using a wearable such as the Apple Watch, which measures HRV at night when you wear it automatically and saves it to Apple Health. The readings will then be synced to the Gyroscope app when you wake up in th emorning, where you can see them in context of your Sleep Score and correlate them to all your other health and lifestyle factors. You will see the HRV values in your last sleep score, as well as longer term patterns in your Trends dashboard.

Other devices like Whoop or Oura track some HRV measurements as well, but do not currently share that data to Apple Health for use by Gyroscope. In those cases, your resting heart rate will be a useful indicator and is generally closely correlated with HRV.

While HRV can also be checked throughout the day, for our purposes the most useful data will come at night while you are sleeping. This is when we actually expect HRV to be high, and can most consistently compare measurements across days. Your HRV at 1pm may be high or low depending on what you were doing — if you were walking or running it would be expected to be low, while if you were relaxing or napping it may be higher — so looking at a graph of daytime HRV would be interesting but much less useful.

For example, during or after a workout, HRV is expected to be low.

Both your average HRV, and the highest number, is useful to understand, and can be easily seen from your Sleep Score soon after you wake up each morning, once your wearables finish syncing and upload the recorded history. Looking at the shape of the graph can be useful too, as there is a circadian component and certain time of night where they would be expected to be highest.

If your device doesn’t track HRV, resting heart rate is the next best thing and is often related. Often a high HRV will be accompanied by a low resting heart rate, which are both correlated to the ANS being in a parasympathetic state.

How can I improve it?

First of all, you may want to figure out core lifestyle issues like getting your sleep battery fully charged, managing chronic stress and other conditions. Once the basics are in place, you can also try out the various experiments in the Labs to improve sleep and see what habits have a positive effect on your HRV.

How do you interpret it?

Your HRV graph is just one metric, and is best interpreted alongside other related signals like resting heart rate, mood, stress levels, activity and more. Together, these can give us a complete picture of your Autonomous Nervous System (ANS), and whether it is in a sympathetic or parasympathetic state.

There is a lot of nuance taken into account by the health score, but a very simplified explanation is that you should expect your HRV to be high at night when resting, and low during a time of intense stress, like while exercising. If your HRV is low during sleep, that is a sign that your body is still in a stress-response state. This is not ideal, and can be a sign that it needs further attention. The other metrics in Gyroscope can then be used to narrow it down further — is it physical stress, mental stress, an illness or immune system activity, or something else?

It's also important to not be too worried if readings are sometimes irregular, or even low. Both your heart rate and HRV will naturally vary significantly through the day as you do things. One of the challenges in the Health Score algorithm is to analyze the data correctly, and final signal in the noise.

Higher HRV readings tend to occur during REM sleep, while lower HRV values tend to occur when you're in deep sleep. The more optimal your sleep habits, then the better you drift through the natural sleep cycles, which will positively affect your HRV score, as well as almost every aspect of your life.

Higher-than-baseline HRV levels tend to be signs of good general recovery and fitness level, but can also be aided by:

  • A much needed day of rest
  • A cool bedroom at night
  • Mindful activities like hiking or yoga
  • Meditation or mindfulness

Lower-than-baseline readings are usually a sign of strain on the body, which could be a result of:

  • Dehydration
  • Alcohol or other toxins
  • A late-night meal or workout
  • Illness or a weakened immune system
  • Mental Stress
  • Sleeping in a hot room
  • Jet lag or circadian disruptions
  • Inconsistent sleep patterns
  • Inflammation
  • Overtraining
  • High bodyfat levels
  • Low cardio fitness

Finally, while high HRV is generally a good thing, if your levels are considerably higher than baseline and you're not partaking in exercise too regularly, this can also be an indication of too little ANS stimulation. While being very relaxed and rested is a good thing, you also do want to push yourself. Your sleep score will indicate when you are well rested and should push yourself further during workouts.

How to raise your RHRV?

Step 1 is go through your health score and check the boxes on all the basics of health. Eating nutritious and well balanced meals, staying active throughout the day, getting high quality sleep, having a healthy body composition, and all the other best practices that are already known to be essential for health.

Beyond that, there are many experiments to try in the Health Labs that can raise your HRV further, especially during sleep. Here are some of the most popular:

  • Mouth tape
  • Eye mask
  • Make it colder
  • Walk after eating
  • Breathwork
  • Sauna

If you're worried about your resting HRV being too low, don't despair!

Exercise, mindfulness, meditation, breathwork, sleep habits and a healthy body composition are some ways you can turn things around.

Tracking and understanding it is the first step. Beyond that, you now have a variety of powerful tools and experiments to start making changes. Within just a week or two you can see meaningful changes and know if it is working.

Both a stronger mind and stronger body will produce long lasting benefits to your nervous system and and heart health in general. It won’t change overnight, but getting your resting HRV up to optimal levels will likely line up with feeling better, having less stress and more energy.