The surprising effect of inconsistent sleep on your body

Going to bed and getting up at the same time every night is one of the best pieces of sleep advice going, and there is a tangible reason behind it.

When your bedtimes and wake times swing around, your body pays for it overnight. We looked at a year of nights from 103,217 people wearing an Ultrahuman Ring.

The more the length of their sleep changed from one night to the next, the faster their heart ran while they slept, and the less it varied from beat to beat. That means a heart working harder through the night, and over years it comes with a higher risk of heart attack and stroke .

But when you go to bed and when you wake up do not matter equally.

Key takeaways

• Sleep consistency is two separate things: when you go to bed, and when you wake up.
• Members whose nights kept changing length ran 5.6 to 7.1 bpm higher overnight heart rate than members whose nights stayed a similar length.
• Bedtime drift on its own came with about half as much, 1.5 to 3.4 bpm.
• You can control when you wake up more easily than when you fall asleep, so start there.

How inconsistant sleep affects heart rate

The analysis  covered 103,217 people, each with a year of their own nights, from July 2025 to July 2026. Nobody was compared against anybody else. We sorted them by how much the length of their sleep changed from one night to the next, then compared average overnight heart rate.

Almost nobody is perfectly regular. Even in the steadiest group, the length of a night still moved by about an hour. In the least steady group it moved by well over two. This is how much faster the heart ran at each step, in members aged 30 to 44, measured against that steadiest group:

| How much the length of your nights varies | How much faster the heart ran overnight | | About an hour | baseline | | An hour and a quarter | 2.6 bpm faster | | An hour and a half | 3.5 bpm faster | | Nearly two hours | 5.5 bpm faster | | Over two hours | 7.1 bpm faster |

It climbed at every step, and it climbed the same way in the other age bands: 5.6 bpm end to end in members under 30, and 6.6 bpm in members aged 45 and over.

Heart rate variability fell too, by 6.2 ms under 30, 4.6 ms at 30 to 44 and 2.3 ms at 45 and over.

Bedtime mattered, but it mattered less

Sort the same members by how much their bedtime moves instead, and you get a smaller version of the same thing. The tenth whose bedtimes drifted most ran 1.5 bpm above the steadiest tenth under 30, 3.0 bpm at 30 to 44 and 3.4 bpm at 45 and over.

The obvious objection is that we have measured one thing twice, because people with wandering bedtimes tend to have nights of wandering length. So we took members whose bedtimes were equally steady and, inside those groups, compared the ones whose nights barely changed length against the ones whose nights changed a lot. Every one of those nine groups showed the same thing: the ones whose nights changed length ran 4.3 to 8.8 bpm faster.

So going to bed at the same time but getting up whenever gets you very little.

How to anchor your sleep schedule

• Get up at the same time every day: You can choose what time you get up but you cannot choose what time you fall asleep. Keep it steady and your nights come out a similar length, even when you get to bed late.
• Get outside soon after you wake: Waking at the same time is much easier when morning light  is helping you do it.
• Treat the weekend as the main source of drift: Ultrahuman data on weekend lie-ins  shows the impact of social jetlag on weekends. Treat your weekend days just like your work ones.
• Try not to play catch up: Seven hours every night will do more for you than a run of nines and fives.

Frequently asked questions

My bedtime barely moves. Why do my nights still change length?

Because when you go to bed is only half of it, and when you wake up changes on its own. An early alarm, a child awake at five, a Saturday you let run long: each one changes the length and leaves your bedtime untouched. We compared members who went to bed at the same time as each other, and the ones who got up at all hours still ran faster.

Is a 6 bpm difference actually a lot?

It is the difference between the extremes of a large group, so most people sit somewhere in between. It is also an average across a whole year of nights, not one bad week. An experiment that steadied sleep on purpose  moved resting heart rate by 3 bpm in twelve nights.

Is this the same as the resting heart rate in my app?

No, and your resting heart rate runs lower. Every figure here is an average across the whole night, 67.0 bpm across these members. Resting heart rate is the lowest your heart reached on that same night, 57.1 bpm. The two sit about 10 bpm apart.

References

1. Ultrahuman. Does an irregular bedtime raise your heart rate? Ask Our Data . 103,217 Ultrahuman Ring members, July 2025 to July 2026.
2. Huang T, Kianersi S, Wang H, et al. Sleep duration irregularity and risk for incident cardiovascular disease in the UK Biobank. Journal of the American Heart Association 2025;14(15):e040027. PMID 40689531 . 86,219 participants, 2,310 cardiovascular events.
3. Gonzales JU, Narvaez G. Short-term reduction in sleep irregularity modulates cardiac autonomic function and central hemodynamic parameters at rest. Sleep Medicine 2025;133:106643. PMID 40543253 . Randomised crossover, 16 adults.

This article describes patterns in Ultrahuman member data. It reports associations rather than cause and effect, and it is not medical advice. If your overnight heart rate has shifted in a way that surprises you, take the trend to a doctor rather than acting on it alone.