What Is Sleep Regularity, and Why Might It Matter as Much as Sleep Length?

Most sleep advice is about how long you sleep. A growing body of research looks at something else: when you sleep, and whether it’s the same from one day to the next. Two people can both average seven and a half hours a night, one asleep from 11 to 6:30 every day, the other from 1 a.m. on weekends and 10 p.m. before an early start. Their averages match; their nights don’t.

This article is general information, not medical advice. If you’re worried about your sleep, talk to a health professional.

How Regularity Is Measured

The most widely used measure is the Sleep Regularity Index (SRI), introduced by Phillips and colleagues in 2017. It compares each moment of a day with the same moment 24 hours later and asks: were you in the same state, asleep or awake, both times?

• 100 means you were asleep and awake at exactly the same times every day.
• 0 means your sleep on one day says nothing about the next, as if your times were random.
• Below 0 is possible in theory, when you tend to be asleep at the times you were awake the day before, but it’s rare.

Because it compares every moment, the SRI notices what an average bedtime misses: a nap one day and not the next, a lie-in at the weekend, or waking for two hours in the night.

What the Studies Found

In the 2017 study, 61 university students kept sleep diaries for 30 days. The least regular sleepers had later circadian timing, with their body clocks running later than those of the most regular sleepers, and a higher SRI went with better grades.

The largest study so far used wrist-worn accelerometers on 60,977 UK Biobank participants, with deaths recorded over about six years. Windred and colleagues found that, compared with the least regular fifth of participants, the other four fifths had a 20% to 48% lower risk of dying of any cause over that time. In their models, sleep regularity predicted mortality better than sleep duration did. The median SRI in the group was 81.

Two cautions: these are observational studies, so they show an association, not that changing your schedule would change your risk; and irregular sleep often comes with things that harm health on their own, like shift work and illness.

In 2023, a National Sleep Foundation panel reviewed the evidence and agreed that consistent sleep and wake times matter for health, safety, and performance. It also noted that when you can’t sleep enough during the week, catching up on days off may still help.

Making Your Sleep More Regular

• Anchor your wake time. A consistent time to get up, weekends included, is often easier to keep than a fixed bedtime, and your bedtime tends to follow.
• Keep weekend shifts small. Sleeping in by an hour moves the SRI far less than sleeping in by three.
• Get daylight early. Morning light helps keep your body clock in step with your day.
• Look at weeks, not nights. One late night is life. A pattern across weeks is what the SRI is measuring.

Following It in Kaizora

Kaizora calculates your Sleep Regularity Index from the spans of sleep your wearable records or you log, naps included. At the end of each sleep, it compares each of the 14 days before with the day before it, on Phillips’s scale from −100 to 100; spans of sleep less than an hour apart count as one sleep. Its page explains how it’s calculated, and its chart shows how your regularity changes over months.

Regularity is also one of the components of the Kaizora Sleep Score, which scores each sleep of 3 hours or more, beside how long you slept, how efficiently, and your share of deep and REM sleep. Like any metric, the SRI can have a reference range of your own, and you can compare it with others in Correlations, such as your resting heart rate. For how far to trust the sleep your watch or ring records, see How Accurate Are the Sleep Stages From Your Watch or Ring?

The Short Version

• Sleep regularity is how closely your sleep and wake times match from day to day.
• The Sleep Regularity Index runs to 100 for perfectly regular sleep; 0 is random.
• In a study of about 61,000 adults, more regular sleepers had a lower risk of death, and regularity predicted it better than duration.
• A steady wake time, small weekend shifts, and morning light are the usual places to start.

Sources

• Phillips AJK, Clerx WM, O’Brien CS et al., “Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing,” Scientific Reports 7 (2017): 3216. doi:10.1038/s41598-017-03171-4
• Windred DP, Burns AC, Lane JM et al., “Sleep regularity is a stronger predictor of mortality risk than sleep duration: a prospective cohort study,” Sleep 47, no. 1 (2024): zsad253. doi:10.1093/sleep/zsad253
• Sletten TL, Weaver MD, Foster RG et al., “The importance of sleep regularity: a consensus statement of the National Sleep Foundation sleep timing and variability panel,” Sleep Health 9, no. 6 (2023): 801–820. doi:10.1016/j.sleh.2023.07.016