HRV during sleep: what is normal overnight

HRV during sleep is the variation between heartbeats measured while you sleep, and it follows your sleep stages. In non-REM sleep, autonomic balance shifts toward parasympathetic (vagal) activity, so vagal HRV is higher. In REM sleep, it looks much like wakefulness (Trinder et al., 2001). Rings and watches average these changes into one nightly number, which is best read against your own recent nights.

HRV (heart rate variability) is a marker of how your autonomic nervous system balances stress and recovery. For daytime ranges and what counts as good, see the pillar guide what is a good HRV.

What is HRV during sleep?

HRV during sleep is heart rate variability recorded while you are asleep, usually by a ring, watch or strap. Because you are still and undisturbed, overnight readings avoid much of the daytime noise from movement, posture, caffeine and conversation. Devices sample HRV in short windows through the night and report an average, sometimes with a peak value and a chart.

Oura's help page explains the reasoning: daytime activities add "noise," which is why it measures HRV during sleep. Oura uses five-minute samples through the night and shows the average of those samples, plus the night's maximum.

Apple says Apple Watch Series 12 includes HRV in its overnight vitals and measures it as often as every five minutes. Other devices use their own windows and formulas, so overnight HRV values are only comparable within one device.

How does HRV change across sleep stages?

HRV changes with sleep stage, not with time of night. In a lab study of 14 healthy young adults over three nights, vagal activity rose abruptly at sleep onset and stayed fairly constant within each non-REM stage. REM sleep looked similar to wakefulness. Changes across the night reflected the shifting mix of stages (Trinder et al., 2001).

State	Autonomic balance	Typical vagal HRV	Heart rate
Awake before sleep	Mixed	Baseline	Higher
Light non-REM (stage 2)	Shifted toward parasympathetic	Higher than awake	Lower
Deep non-REM (slow wave sleep)	Parasympathetic dominance	Higher than awake	Lower
REM sleep	Similar to wakefulness	Closer to awake levels	Closer to awake

Source: Trinder et al., J Sleep Res 2001 (stages and balance); Stein and Pu, Sleep Med Rev 2012 (review of HRV and sleep stages).

Trinder and colleagues found one exception to the "stage not time" rule: heart rate itself also showed a time effect through the night, which they linked to a circadian influence.

What is a normal overnight HRV?

There is no single normal overnight HRV, because devices differ in metric, window and averaging. Oura reports typical values that range "from under 20 to over 200 milliseconds" across people. A normal overnight HRV is one that sits inside your own recent range on the same device, measured on ordinary nights.

What shapes your number:

Factor	Effect on overnight HRV
Age	Lower with age, fastest for RMSSD until about 60
Device and metric	RMSSD and SDNN give different values
Sleep stage mix	More non-REM time tends to raise the average
Alcohol	Lower in the first hours of sleep, dose dependent
Sleep loss	RMSSD lower after sleep deprivation

Sources: Pietila et al., 2018; Zhang et al., 2025; Trinder et al., 2001. Age data: see HRV chart by age.

Overnight values can be quite different from a short morning reading. Do not compare an overnight ring average with a short camera reading or with a lab table. Each method needs its own baseline.

Why is my HRV low during sleep?

A low overnight HRV is most often associated with alcohol, short sleep, a hard training day or the start of an illness. Alcohol has the clearest data: in 4,098 adults, RMSSD in the first 3 hours of sleep fell 2.0 ms after about one drink and 12.9 ms after about seven (Pietila et al., 2018).

Cause	What the research shows	Source
Alcohol before bed	RMSSD 2.0 to 12.9 ms lower in the first 3 hours of sleep, by dose	Pietila et al., 2018
Sleep deprivation	RMSSD significantly lower (11 trials, 549 people)	Zhang et al., 2025
Hard training day	Next-morning RMSSD about 4.8% lower	Altini and Plews, 2021

Source: as listed per row.

Most adults need 7 or more hours of sleep per night (Watson et al., 2015). The full alcohol data is in alcohol and HRV. For causes beyond sleep, see low HRV meaning.

Is overnight HRV better than a morning reading?

Neither is better in general. Overnight HRV needs no effort and averages many samples, but it depends on your sleep stage mix. A morning reading is controlled and quick, but needs a daily habit and the same position each time. Pick one method, use it every day, and compare only like with like.

	Overnight average	Morning spot reading
Effort	None, wear the device	1 to 5 minutes each morning
Samples	Many windows through the night	One window
Main influences	Sleep stages, alcohol, bedtime	Position, breathing, time after waking
Devices	Rings, watches, straps	Chest strap or phone camera

To try morning readings without a watch, see measure HRV with your iPhone camera.

Can HRV during sleep show sleep problems?

HRV during sleep has been used in research on sleep disorders. Stein and Pu (2012) review studies applying HRV to sleep-disordered breathing, periodic limb movements and insomnia, and note that HRV "has been successfully used to screen people for possible referral to a Sleep Lab." A consumer wearable cannot diagnose these conditions.

See a doctor if you snore loudly, stop breathing or gasp during sleep, wake up many times a night, or feel very sleepy during the day. These need a proper sleep evaluation, not an app score.

How do you improve HRV during sleep?

To support a higher overnight HRV, give yourself enough sleep, keep a regular schedule, and avoid alcohol close to bedtime. Alcohol and sleep loss are both associated with lower RMSSD in the studies above. Over weeks, regular aerobic exercise is associated with higher resting HRV in training studies.

Practical steps:

1. Aim for 7 or more hours in bed asleep.
2. Keep bedtime and wake time within a narrow window.
3. Keep alcohol-free nights, especially before important days.
4. Plan easier days after hard training.

More levers are in how to increase HRV.

How Svanu uses overnight data

Svanu reads HRV, resting heart rate and sleep from Apple Health, so it works with an Apple Watch and with other devices that sync there. The recovery score compares this morning's HRV and resting heart rate with your recent baseline, plus sleep, and shows what moved it.

The sleep score runs from 0 to 100: duration 50%, stage quality 30% and bedtime consistency 20%. The sleep screen shows stages, sleep debt and tonight's bedtime. If your device does not write HRV, recovery uses resting heart rate and sleep. See the Svanu home page for details.

The short answer

HRV during sleep rises in non-REM sleep and falls toward waking levels in REM and brief awakenings. Your nightly average reflects that mix plus factors like alcohol and sleep loss. There is no universal normal overnight number. Compare each night with your own range on the same device.