Does an irregular bedtime raise your heart rate?

A drifting bedtime does show up in your heart rate: within each age band, the tenth of members whose bedtimes moved most ran 1.5 to 3.4 bpm above the tenth whose bedtimes barely moved. But bedtime turns out to be the smaller half of the story. Sort the same members by how much the length of their nights changes — seven hours, then five, then nine — and the gap roughly doubles, and it survives even among members whose bedtimes are equally steady.

Which kind of sleep consistency shows up in your heart rate?

Sleep consistency has two halves. The first is your bedtime, the clock time you fall asleep: 11pm one night, 2am the next. The second is how long the night then runs: seven hours, then five, then nine. They are not the same thing — you can go to bed at eleven every night and still get all three of those lengths, because your wake time sets the other end. Both move with overnight heart rate. Sleep duration moves with it more, and it is the half this analysis follows.

Phillips and colleagues built the Sleep Regularity Index, which scores the chance of being in the same state, asleep or awake, at any two points 24 hours apart [1]. Among 60,977 UK Biobank participants, the most regular sleepers were less likely to die during the study, whatever the cause [2]. Chung and colleagues, in 1,726 adults followed over a median of 6.9 years, found regularity and sleep length among the largest drivers of a combined sleep score's link to death rates [3]. Figure 1 shows the second half, how the length of the night changes.

A second overnight marker moved in step. Heart rate variability — how much the gap between one beat and the next changes across the night, which usually falls as the heart speeds up — ran lower in the most variable tenth of each age band than in the least variable: 6.2 ms lower under 30, 4.6 ms at 30 to 44 and 2.3 ms at 45 and over. The night-to-night swing in sleep length behind those groups was 59 against 145 minutes under 30, 55 against 139 at 30 to 44, and 52 against 139 at 45 and over.

Does this hold when bedtimes are equally steady?

Bedtime and sleep duration travel together, so the fair question is whether the duration gap is just bedtime measured a second way. We took members whose bedtimes were equally steady, and inside that group set the ones whose sleep duration barely changes against the ones whose duration changes a lot. The gap held, 4.3 to 8.8 bpm, in all nine groups: three age bands by three levels of bedtime steadiness. Figure 2 shows all nine.

Longer studies point the same way. In 86,219 UK adults who wore a movement tracker for a week, every extra hour of night-to-night variation in sleep duration came with a 19% higher chance of a heart attack or stroke over the years that followed [4] — the same measure we used, followed forward to hard events. The one experiment we have is small: sixteen adults kept a set sleep schedule for twelve nights and their usual one for twelve, and on the set schedule their sleep duration varied by 31 minutes instead of 57, their average night was no longer, and their resting heart rate fell from 65 to 62 bpm [5]. Sixteen people is not many, and a set schedule steadies the clock as well as the duration, so it cannot tell those two apart. It does show that steadying sleep moves heart rate, in the direction we measured.

Is overnight heart rate the same as your Ring's resting heart rate?

Your Ring's resting heart rate is a different number, and it runs lower. Every heart rate quoted so far is a night average, 67.0 bpm across these members. Resting heart rate is the lowest your heart got on that same night, 57.1 bpm. The two sit 9.9 bpm apart and rise and fall almost in step. A faster resting heart rate does travel with higher death rates across 46 cohorts and more than a million people, though the authors report substantial heterogeneity and publication bias [6] — and that rate was taken awake in a clinic, not from a sleeping body.

What this analysis can't tell you

Members appear once and nobody is followed forward, so nothing here says what happens when you steady your own sleep. Writing on 82,391 UK adults, Bian and colleagues open their own long-run conclusion with “Assuming that the associations we observed are causal” [7]. A night two hours longer or shorter than the one before could be driving the faster heart, or could be a sign of something else that is. Shifting sleep about twelve hours out of phase in a laboratory changes blood pressure and glucose handling [8] — far heavier than anything these members lived through. And Ultrahuman members watch their numbers, so this group is likely steadier than most.

Frequently asked questions

My bedtime barely moves. Why does the length of my nights still change?

Bedtime is one end of the night, and the other end moves on its own. An early alarm, a child awake at five, a Saturday you let run long: each changes the length and leaves the bedtime alone. Compare members with equally consistent bedtimes and the gap is still there, in all nine groups.

Why would the length of a night change what my heart does overnight?

We cannot say why, only that a second measure moves with it. Heart rate variability is how much the gap between one beat and the next changes across the night. Across the ten groups in Figure 1 it ran 6.2 milliseconds lower in the most variable tenth under 30, 4.6 at 30 to 44 and 2.3 at 45 and over. In these members the heart beat faster, and its rhythm varied less.

Who should not read this as being about them?

Anyone whose nights are set by someone else. Shift work, a newborn, illness and long-haul travel all stretch and shorten nights, none measured here. Anyone diagnosed with a heart rhythm or breathing condition is reading a number we never described. A rate far from your baseline may be worth a conversation with your clinician.

References