Modern smart watches can measure all sorts of health indicators. Step counts and heart rates sit at the simpler end of the array, whereas VO2 max and blood-oxygen levels are of more interest to committed health nuts. But a category currently attracting particular attention is heart-rate variability (HRV).
As its name suggests, HRV measures not how quickly the heart beats, but how regularly spaced those beats are. With heart rate a lower score is usually better, other things being equal, since it suggests a high level of cardiovascular fitness. When it comes to HRV, though, a higher number—that is, a more irregular pattern—is generally what you want.
HRV arises from the way the body regulates the heart. Left to its own devices, the heart will chug along at 100 beats per minute or so. That default rate is nudged up or down by the opposing halves of the autonomic nervous system (ANS), which acts unconsciously to control things like body temperature, breathing and digestion.
One half is the sympathetic nervous system, often known as the “fight-or-flight” system. This revs up the heart in response to things like exercise or fear or excitement. Its control is mostly exerted through hormones in the blood and neurotransmitters in the brain. That makes it a blunt instrument, and as the heart rate rises, so the time between beats becomes steadier.
The other half of the ANS is the parasympathetic, or “rest-and-digest” system. This slows the heart down when it is time to relax. It communicates with electrical signals sent via the vagus nerve. That allows precise control from moment to moment, which makes the time between heartbeats more variable. (Your heart rate speeds up slightly when you breathe in, for instance, and slows down as you breathe out.)
All else being equal, stress on the body boosts the sympathetic nervous system, decreasing HRV. All sorts of stress count, whether psychological or physical. A hard workout will cause HRV to fall for hours (or sometimes days) as your system recovers. So will lack of sleep, a cold, a failing marriage or worries about money.
At a population level, higher HRV is a sign of an ANS that is in good nick, and a body that can adapt itself to the stresses of life. It is associated with a lower risk of heart attacks, and a higher chance of survival if you do have one. (It was in cardiology wards that HRV first proved its usefulness.) It is also associated with slower progression of dementia, less inflammation, a lower chance of suffering depression, and more.
Most people will be more interested in what HRV can tell them about their personal lives. The best way to think of it is as an “accumulated stress” score. For sporty types a low HRV may be a signal that the body is struggling with too much training, and a hint to go easier in the gym for a while. But the numbers need careful interpretation. A big day out on the bike will cause a low HRV in the morning. But so will beers at the pub afterwards (alcohol suppresses HRV), or an unexpected tax bill waiting at home.
Those nerdy enough to track HRV may therefore want to keep a journal to refer to alongside the charts. Knowing what the numbers are telling you, after all, is the difference between mere data and its much more valuable cousin—useful information. ■
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