How long do changes in sleeping heart rate last around an illness?

Changes in sleeping heart rate around a logged sick day lasted a few nights in this group. On the night just before the sick day, the group's average sat 4.4 bpm above each member's own baseline (likely between 3.9 and 4.9 bpm), across 946 members. The gap shrank on each of the next nights. Four nights after the night of the sick day, it was 0.2 bpm, and its likely range includes no change.

What happens to sleeping heart rate around a logged sick day?

Sleeping heart rate in this group stayed near baseline for most of the two weeks before a logged sick day, then climbed over the last few nights before it.

The gap was still clearly above baseline on the night of the sick day. A short rise and return has also been seen after vaccination. In most smartwatch users given an mRNA COVID-19 vaccine, resting heart rate rose above their usual level, peaked on day 2 and was back to normal by day 6 [1].

A logged sick day was a calendar day the member tagged as Sick, Sick Day, Cold/Flu or Illness by hand in the app. The tag had to be saved before the app first recorded the following night's sleep. Illness labels the app works out from heart or temperature readings were not used, and neither were symptom tags such as Sore Throat. Only the first logged day of an illness counted: a day with another of those four tags in the 28 days before it was left out, and counted days from the same person were at least 43 days apart, so no night served two comparisons. The personal baseline was the member's average sleeping heart rate over 14 nights that ended two weeks before the logged day, requiring at least 10 valid nights recorded on the same ring and firmware version. Ending it two weeks early keeps the nights just before the logged day out of the baseline, so a rise in those nights shows up instead of being averaged away.

Each night from 14 before the logged day to 14 after it was compared with that baseline. Night zero was the sleep that ended on the morning of the logged day, the night just before the sick day. Night one was the night of the sick day. A night counted only with a valid reading on the same ring and firmware version, so each night covers a slightly different group of people. Nights without a sick tag were not treated as healthy nights, and nobody recorded when an illness began or ended. Each logged day is compared only with the same person's own nights before it. Members who logged more sick days in the two weeks that followed stayed in.

Each person counted equally, however many illnesses they logged; the uncertainty ranges resampled whole people.

Does sleeping heart rate rise before the sick day is logged?

Yes: sleeping heart rate in this group was already above baseline before the logged day, including two nights before the sick day. Those nights may already be part of the illness. Some members may also have logged a tag after the app flagged a raised reading. When illnesses with a recent app alert were left out, the rise before the logged sick day was smaller. In a smartwatch study of people infected with COVID-19, most of those with detected changes in heart rate, steps, or sleep were detected before or at symptom onset [2].

How long does it take to get back to normal?

Sleeping heart rate in this group stayed above baseline from the night of the sick day through the third night after it. On that third night it was still 0.5 bpm above baseline (likely between 0.1 and 0.9 bpm). The night after that, the gap was too small to tell apart from no change. The chart also shows small rises on two nights in the second week, which may be chance, and the analysis cannot say why they appear. Heart rate variability (HRV), a different heart measure, has been studied after coronavirus vaccination, a different trigger from illness. A review of observational studies found short-term changes and rapid recovery in heart rate variability within up to three days after COVID-19 vaccination [3].

What this analysis can't tell you

Illness here means a member chose to log a sick tag in the app, so nights without a tag are not known to be free of illness. A tag records the day, and not always the hour. No test confirmed an infection, so the curve is timed to the day a member chose to log, which may come after the illness began. Members who log sick days may differ from members who never do. The figures are group averages, so they do not show how any one person recovered. The analysis had no record of anything else that happened in those weeks, so it cannot separate other causes from the illness and does not measure a cause.

Frequently asked questions

Is sleeping heart rate the same as resting heart rate?

No: sleeping heart rate is your average heart rate across the whole night's sleep. Resting heart rate is a separate measure, covered in Ultrahuman's resting heart rate explainer, so the numbers here don't apply to it.

Does a fever explain the rise in sleeping heart rate?

Not from this analysis: a logged tag does not record whether a member had a fever. In 27 young men with an acute febrile infection, heart rate rose by about 8.5 beats per minute for each 1 °C rise in temperature [4]. The rise here may include nights with a fever and nights without one, and the group average cannot separate them.

Does this apply to a serious or long illness?

This finding covers logged illnesses, so it says little about a hospital stay or an illness lasting weeks. Among survivors of COVID-19 critical care who wore a smartwatch, mean daily resting heart rate was lower one year after discharge than in the first month [5]. A review of cross-sectional studies found lower heart rate variability in people who had recovered from COVID-19 than in controls [6]. Neither measure is sleeping heart rate, and this analysis followed only 14 nights after the logged day.

References