April 3, 2020

You’ve probably heard or read stories about Garmin watches
and similar wearables seemingly predicting illness. Can your fitness tracker
really tell when you are getting sick? Or are wearable enthusiasts reading too
much into the data?

Let’s start with the critical caveat.
Garmin watches are not approved
medical devices and are not designed to diagnose or treat any disease or
condition. Always seek professional advice if you are concerned about
your health.

Your body has a certain amount of resources at its disposal to do work, like exercising, planning a project or cooking dinner. These resources are like the energy in a rechargeable battery, and that battery is unique to you. Any kind of stress, physical or mental, drains your battery, while recovery, like sleeping, recharges it.

Garmin’s Body Battery™ uses Firstbeat analytics to measure
stress and recovery to tell you how your personal battery is doing, but how
does this work in practice? Your body’s complex nervous system has a particular
area called the autonomic nervous system (ANS). The ANS, as the name implies,
runs certain bodily functions automatically, like digestion, heart rate and
normal breathing. [1]

The ANS is where stress and recovery are controlled through
to sub-systems, the sympathetic and parasympathetic. The sympathetic nervous
system is the source of our instinctual “fight or flight” response to stress.
This is our body preparing to work. The parasympathetic nervous system, on the
other hand, steers our “rest and digest” functions.  [1]

What your heart can tell you

Your heartbeat is closely regulated by your ANS, determining
both heart rate and heart rate variability (HRV). HRV is the variation of time intervals
between heartbeats. The difference is in milliseconds, so we typically don’t
notice changes in HRV like we do with heart rate (beats per minute).

This means that analyzing how your heart beats from one moment
to the next can reveal transitions between fight-or-flight and rest-and-digest
states. When you are under stress, your heart rate goes up, and your heart rate
variability goes down. We may notice the former, but it is the latter that
reveals the most about your current state.

Stress, like exercise, raises heart rate and lowers HRV.
When you are relaxed and resting, your heart rate goes down while HRV goes up.
Think of it this way: your heart is a finely tuned engine that revs only as
much as it really needs, and that tuning is so good that it can change revs
from one second to the next.

Now, let’s look at this in terms of who’s in control. Low
HRV indicates stress, and stress means your sympathetic system is running the
show. High HRV indicates rest, and rest means your parasympathetic system is at
the wheel. [2]

When everything is in balance, your Body Battery shows a regular cycle of draining and charging over time. That cycle can get disrupted, causing, for example, faster draining during the day, or slow recharging during the night. One such disruption could be illness.

What happens when you are sick?

Illness kicks off an immune response, which is largely
driven by hormones. These hormones tell the body to start producing more white
blood cells, for example, and they also trigger the ANS to increase sympathetic
activities. This is what drives heart rate, sweating, etc. as your body works
to fight the illness. Physiologically, the body senses stress and sympathetic
nerves take over. HRV usually starts to decrease slowly, though there are
variations; more severe illness may have a bigger impact faster. [3]

Early on, it is difficult to say if a low battery is because
of illness or something else. If you also have a slight headache, sore throat
or unusual sneezing, for example, then you are likely getting sick. When you
are getting sick, you may also see higher than normal heartrate and training
load in your regular training sessions.

Note that fever is a major
stressor on the body. A 1 °C increase in your temperature can raise your average
heart rate by over 8 beats per minute. [4]
Physiological imbalances may also be displayed as higher stress levels. For
example, unbalanced diabetes or hypo- or hyperthyroidism can be reflected in
your stress charts [1]
and lower body battery values.

Staying healthy

Your body’s stress response is about being prepared for challenges.
It gives you the energy to work hard and focuses the mind. In the same way, straining
your muscles and cardiovascular system improves your fitness, making you better
prepared for the next physical challenge.

Facing down challenges drives us to adapt and grow stronger.
Stress only becomes unhealthy if it continues for too long or is too high. [5] Excessive stress levels
over time diminish our ability to bounce back from challenges and weaken our
immune systems. [6] open the door to.

A balanced exercise program and taking care of yourself by eating
and sleeping…