Is HRV higher in deep sleep or REM?
In deep sleep. In lab studies, the part of HRV that tracks the vagus nerve runs higher in deep, non-dream sleep than in REM, and REM looks more like being awake. That’s one reason a ring and a band that weight different parts of the night give different numbers.
People picture sleep as one long rest. Your heart doesn’t. Through the night you cycle between light sleep, deep sleep and REM, the dreaming stage, and the heart’s rhythm changes with each.
Think of a night as a drive through town and country. Deep sleep is the open road, where the heart settles into a slow, easy, breath-by-breath sway. REM is back in town, busier and more like the daytime. Your night’s average depends on how much of each road you drove.
What we’d do: don’t chase deep sleep for a better number. Keep your sleep regular and judge each night against your own usual range. A night with less deep sleep can come in a little lower, and that’s fine.
What the studies show
| Study | Who | Finding |
|---|---|---|
| A 2001 sleep lab study, as summarized by two WHOOP scientists in 2025 | Sleeping adults in a lab | Heart rate 3.5 percent lower and a vagal HRV measure 123.7 percent higher in deep sleep than in REM. They note the exact size varies between studies |
| Daytime naps, 2016 | 66 healthy young adults | Slower heart rate and higher vagal HRV in non-dream sleep than in REM or awake |
The swing comes mostly from breathing. A 2017 review describes the breath-linked rise and fall of heart rate, driven by the vagus nerve, as the main source of HRV at rest.
Why it matters for your device
| Device | Which part of the night |
|---|---|
| Oura | Five-minute samples across sleep, averaged for the night. The app also shows the night’s highest value |
| WHOOP | An average weighted toward your last deep sleep stage |
| Garmin | Five-minute windows averaged across sleep |
Because WHOOP leans on deep sleep and Oura averages the whole night, the same person on the same night can get two different RMSSD values. Neither is wrong. They’re looking at different stretches of the same night.
5 sources
- Grosicki GJ, Presby DM, Accurate comparison of wearables requires contextual equivalence, Physiological Reports, 2025pubmed.ncbi.nlm.nih.gov
- Cellini N et al., Heart rate variability during daytime naps in healthy adults: autonomic profile and short-term reliability, Psychophysiology, 2016pubmed.ncbi.nlm.nih.gov
- Dial MB et al., Validation of nocturnal resting heart rate and heart rate variability in consumer wearables, Physiological Reports, 2025pubmed.ncbi.nlm.nih.gov
- Oura Help, Heart Rate Variability, updated 2026-10-01support.ouraring.com
- Shaffer F, Ginsberg JP, An overview of heart rate variability metrics and norms, Frontiers in Public Health, 2017pubmed.ncbi.nlm.nih.gov
