Sleep changes your heart rate variability (HRV) in two ways: it shifts through the night as you move between sleep stages, and a run of short nights tends to lower it. That is why most wearables measure HRV while you sleep, and why your number is easiest to read when you compare it with your own usual range rather than with last night alone.
Why HRV changes while you sleep
Your heart rate is steered by two branches of the nervous system. The parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system branch, mostly through the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve , slows the heart and lets the gap between beats vary more from one beat to the next. The sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system branch speeds the heart up and makes the rhythm steadier. HRV goes up when the parasympathetic side has more say.
Sleep is not one steady state. You cycle through light non-REM sleep, deep (slow-wave) sleep and REM sleep several times a night, and each stage has its own pattern:
- Non-REM sleep. As you fall asleep and move into deeper non-REM sleep, the balance shifts toward the parasympathetic side. In a study of 14 young, healthy adults measured over three nights, this shift happened abruptly at sleep onset and held steady within each stage (Trinder and colleagues, 2001).
- REM sleep. During REM sleep, the dreaming stage, the balance in the same study looked much like being awake. A later study found REM sleep tilted toward more sympathetic activity (Boudreau and colleagues, 2013).
- Time of night. The body clock adds its own rhythm on top. In the Boudreau study, parasympathetic activity during slow-wave sleep peaked at around 2 a.m., while sympathetic activity during REM sleep peaked in the early morning.
So the mix of stages in a night can move your average HRV, even if you feel the same in the morning. Your device averages across all of this, which is one reason the number changes from night to night.
Go deeper How the sleep-stage studies were done
Trinder and colleagues recorded 14 young, healthy people on three nights, scored their sleep stages and measured HRV in 2-minute blocks. The autonomic balance changed with sleep stage rather than drifting with time spent asleep. Heart rate was the exception: it also followed the body clock.
Boudreau and colleagues kept 13 healthy young people in time isolation for 72 hours, alternating 60 minutes awake with 60-minute nap opportunities. This let them separate the effect of sleep stage from the effect of time of day. Both studies were small, so they describe typical patterns, not what your own night will look like.
A 2012 review by Stein and Pu summarises how HRV is used in sleep research, including its links with sleep apnea and insomnia, and lists the pitfalls in reading it.
What a short night does to HRV
Sleep loss pulls HRV down. In a Swiss study, 15 healthy adults (eight women, seven men) slept normally for a week, then slept only 3 hours a night for three nights, then recovered with as much sleep as they wanted (Bourdillon and colleagues, 2021). Each morning they did a short lying-then-standing test while wearing a chest-strap heart monitor. During the short-sleep days, RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD and high-frequency power HF power The share of heart rate variability in fast rhythms of 0.15 to 0.40 Hz, the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power fell, which the authors read as less parasympathetic activity. The participants also felt more tired and sleepy.
Three hours a night is extreme, and the study was small. Still, if you sleep much less than usual for a few nights, your HRV may well drift down.
Other things that show up in your night number
Your overnight HRV also carries traces of the day before. Things that often push it down include:
- alcohol, which lowers HRV in the first hours of sleep
- caffeine late in the day, mainly by cutting into sleep
- a hard training session
- the start of an illness
Sleep problems also leave a mark. Researchers have used HRV to study sleep apnea, where breathing stops and starts during the night, and insomnia (Stein and Pu, 2012). A wearable cannot tell you whether you have either. If you snore loudly, wake up gasping, or feel very sleepy during the day even after a full night in bed, talk to a doctor.
How to read your own sleep and HRV data
A single low night tells you very little. These habits make the number more useful:
- Compare with your baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline . Look at how tonight sits against your own range over the past one to four weeks. The lesson on building your HRV baseline shows how.
- Watch the trend. A weekly average smooths out the stage-by-stage noise. One bad night is not a trend; three or four low nights in a row are worth noticing.
- Tag your days. Note late drinks, late meals, travel, a hard workout or a stressful day. After a few weeks you will see which ones move your number.
- Keep the method the same. Overnight and morning readings give different numbers. Read more in overnight vs morning HRV.
- Check how you feel. If your HRV is low but you slept well and feel fine, the number may be at the low end of your normal spread.
If your number swings a lot and you are not sure why, see why your HRV changes so much.