A single night far above your usual range is more often a data event than a body event. The three usual causes are a bad beat in the recording, a change in which hours the device averaged, and a nap or an unusually staged night. A real rise lasts several nights and comes with a normal or lower resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate .
One bad beat is enough
Most devices report RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD , which squares the change from each beat to the next. A beat the sensor misses, or an early (ectopic) beat followed by a longer pause, creates two huge differences in a row, and squaring makes them dominate the whole night. Ciccone and colleagues point out that RMSSD is mathematically the same quantity as SD1 SD1 and SD2 Two measures of the spread of dots in a Poincaré plot, in milliseconds. SD1 is the width and tracks short-term variability, like RMSSD; SD2 is the length and tracks overall variability. More about SD1 and SD2 , the width of a Poincaré plot Poincaré plot A scatter plot of each heartbeat interval against the one after it, in milliseconds on both axes. The shape of the cloud of dots shows short-term and overall variability at a glance. More about Poincaré plot : it measures nothing but beat-to-beat jumps, which is exactly what an artifact Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact creates.
Makers clean beats before averaging, with filters they do not fully publish. In a 2021 validation of WHOOP, Bellenger and colleagues found that the strength of the filter used to remove bad beats changed how closely the device’s RMSSD matched an ECG ECG A recording of the heart's electrical activity, taken with electrodes on the skin. ECG-based RR intervals are the reference standard for measuring HRV. More about ECG , so a night with more movement, a looser strap or a few extra beats can slip through differently from a calm one. The SDNN and RMSSD widget has an “One extra beat” preset that shows the jump on twelve beats.
Different hours, different number
Devices do not all average the same part of the night. In the 2025 overnight study by Dial and colleagues, Oura and Garmin average 5-minute samples across the sleep period, Polar uses the first four hours, and WHOOP’s number is a dynamic average weighted towards the last slow-wave sleep stage of the night. The model below draws one night and each window.
Same night, five numbers
A teaching model, not a measurement
Sleep stages lights out to waking
HRV every 5 minutes (RMSSD) 20 to 60 ms
- Oura
- 47.5 ms RMSSD
The mean of 5-minute samples taken while asleep, across the whole night.
Same night, same heart. The four RMSSD devices report between 44.8 ms (Polar) and 54.6 ms (WHOOP) because they average different hours. Apple Watch's spot readings run from 48.7 to 78.9 ms SDNN, a different statistic again.
The buttons work when JavaScript is on. The windows for all five devices are listed in the description.
Two charts over eight hours of simulated sleep. The top chart is a hypnogram stepping between awake, REM, light and deep sleep, with deep sleep mostly in the first half and REM mostly in the second. The bottom chart is a line of HRV values, one per five minutes, that rise gently through the night and dip during wakeful minutes. Five buttons, one per device, shade the part of the night that device averages. Oura: 47.5 ms RMSSD. Garmin: 47.1 ms RMSSD. WHOOP: 54.6 ms RMSSD. Polar: 44.8 ms RMSSD. Apple Watch: 4 readings, 48.7 to 78.9 ms SDNN; on waking 78.9 ms. Same night, same heart. The four RMSSD devices report between 44.8 ms (Polar) and 54.6 ms (WHOOP) because they average different hours. Apple Watch's spot readings run from 48.7 to 78.9 ms SDNN, a different statistic again.
Two things follow for spikes.
Editing the sleep window moves the number. If you correct your bedtime or wake time in the app, the device recalculates with different hours. For a whole-night average that shifts the number a little. For a number weighted towards the last deep-sleep period, moving the end of the night can swap one deep-sleep bout for another and change the value by far more. If your HRV changed after you edited sleep, that is why.
How the night was staged matters. A late bout of deep sleep, a long lie-in, or a night broken by a wake-up changes which minutes count. The same heart, averaged over a different window, gives a different number. Why Oura and WHOOP show different HRV goes further into the windows.
Naps and long sleeps
A nap can raise HRV for a while. In a randomised trial of 20 trained adults, Besson and colleagues found that a conventional nap raised log RMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD measured afterwards lying down (p = 0.02), a sign of more 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 activity. Devices treat naps differently. Oura counts a nap of 15 minutes to three hours towards its Sleep and Readiness Scores Readiness score A single number, often out of 100, that a device builds from your HRV, resting heart rate, sleep and other inputs to suggest how ready you are for the day. The weighting is not published. More about Readiness score , and a nap recorded after 6 PM updates the scores the next morning. So a nap can raise a score without any change in the night’s HRV, or the night’s HRV can look high because a long sleep included hours of deep rest. Check the sleep timeline before you read the number.
When a high night is real
Rest raises HRV, and that is good news when it lasts. A rise can also come with deep fatigue. In Le Meur and colleagues’ study of 21 triathletes, the 13 who did three weeks of overload training lost about 9% of their running test performance, yet their weekly average HRV rose compared with the control group, reversing in the taper. The authors note that isolated once-a-week readings did not show the effect, because day-to-day variability is large. The page on why your HRV is suddenly high covers that case.
Use three checks:
- Does it last? One night moves a 7-day average by a seventh of the jump. A real change shows in the average over several nights.
- What did resting heart rate do? A spike in HRV with no change in resting heart rate, or with a higher one, is suspect. Recovery usually brings a lower resting heart rate with it.
- What did the trace look like? If your app shows the beats, one isolated jump is an artifact.
When to talk to a doctor
A spike in a wellness number is not a reason to worry on its own. Talk to a doctor if your device warns of an irregular rhythm, or if you notice palpitations, skipped beats, dizziness or chest pain, whatever your HRV shows. Can HRV detect heart problems? explains what the number can and cannot tell you.