# Overnight heart rate variability (HRV)

The variation in time between heartbeats, a window on the nervous system’s recovery side. Only the change against your own baseline on the same device means anything.

Type: biomarker · Unit: ms · Domain: Cardio-Respiratory Health · Accuracy: Trend-grade · Importance: Core

Also called: sleeping HRV, nightly HRV, morning HRV, heart rate variability, recovery HRV, RMSSD

## In one sentence

How much the gap between one heartbeat and the next varies while you sleep, which is a window on the nervous system's recovery side.

## Why it matters

A resting heart does not tick like a clock. Each breath in lets the heart speed up a little and each breath out lets it slow, and the size of that swing is set by the vagus nerve, the brake the parasympathetic nervous system keeps on the heart. More brake, more variability: a high overnight HRV usually means a well-recovered nervous system, and a fall against your own usual means the brake has come off, for a reason that is usually in your record. The level is personal and falls with age across populations (correlation −0.35 in 28,175 people; Altini 2021), which is why only the change against your own baseline, on the same device, means anything.

## What moves it

Downward on a night: heavy alcohol, by about 12 percent in 9 million free-living readings; being unwell, by about 10 percent; the second half of the menstrual cycle, by about 3 percent; a training day, by about 5 percent (Altini 2021). Also downward: a late large meal, a hot room, dehydration, a stressful day, poor sleep. Upward over months: aerobic fitness, better sleep, a slow-breathing practice (see [*Protocols*](https://vitalitytracker.app/knowledge/protocols-how-to)). In very fit people the relationship can invert, with HRV falling as the resting rate falls, a saturation effect first described in elite endurance athletes (Plews 2013), so a lower figure is not always a worse one.

## How to measure it well

The same ring or band every night; nothing else. Read a rolling average rather than a night, because HRV is noisier than heart rate: the research practice is a seven-day average, and the elite-sport practice is to look for a trend across a week rather than a dip on a day (Plews 2013). A night's HRV is also measured over a few minutes at a time, and the method differs between makers (see [*Daily average HRV*](https://vitalitytracker.app/knowledge/daily-average-hrv)), so VT labels every HRV figure with its method.

## How Vitality Tracker uses it

The first input to the recovery side of Capacity, compared with your own usual; a lower-than-usual night pulls the score down, together with the overnight resting heart rate and the night's duration and regularity. It is not one of Symptom Radar's readings; Symptom Radar uses the daily average HRV instead. A change of device starts a new series.

## How far to trust the number

Trend-grade. The measurement itself is good: a finger ring read the nightly average HRV within 1.2 milliseconds of an electrocardiogram, with a correlation of 0.98, in 49 adults (Kinnunen 2020). What limits it is the biology: a single night can swing by more than any one stressor moves it, so the direction of the weekly trend is the usable part, and a single night's number is not.

## Method

Rings and most bands report RMSSD, the root mean square of successive differences between heartbeats, averaged across five-minute windows through the night; it reflects the parasympathetic side almost alone (Shaffer 2017). Stored as the morning HRV with the device and method; Capacity compares it with your own recent distribution.

## Sources

1. Altini M, Plews D. What is behind changes in resting heart rate and heart rate variability? Sensors 2021;21:7932. [doi:10.3390/s21237932](https://doi.org/10.3390/s21237932)
2. Plews DJ, Laursen PB, Stanley J, Kilding AE, Buchheit M. Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Med 2013;43:773–81. [doi:10.1007/s40279-013-0071-8](https://doi.org/10.1007/s40279-013-0071-8)
3. Kinnunen H, et al. Accuracy of nocturnal HR and HRV assessed via ring PPG in comparison to medical grade ECG. Physiol Meas 2020;41:04NT01. [doi:10.1088/1361-6579/ab840a](https://doi.org/10.1088/1361-6579/ab840a)
4. Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health 2017;5:258. [doi:10.3389/fpubh.2017.00258](https://doi.org/10.3389/fpubh.2017.00258).

## Related

- [Daily average heart rate variability](https://vitalitytracker.app/knowledge/daily-average-hrv)
- [Overnight resting heart rate](https://vitalitytracker.app/knowledge/overnight-resting-heart-rate)
- [Capacity: how much you can take on today](https://vitalitytracker.app/knowledge/capacity)
- [Protocols: log the things you do to yourself](https://vitalitytracker.app/knowledge/protocols-how-to)
- [Perceived stress (PSS-4) and the daily stress question](https://vitalitytracker.app/knowledge/perceived-stress-pss4)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/overnight-hrv