Sleep Regularity Index (SRI) Explained: Why Consistent Timing Counts

Two people can both average seven hours of sleep and keep very different schedules. One sleeps from about 11pm to 6am most days. The other is asleep by midnight on work nights, stays up until 3am at the weekend and then has a long lie-in. Their totals look alike; their timing does not. The Sleep Regularity Index, or SRI, describes that difference.

What the Sleep Regularity Index measures

SRI asks whether you are in the same state, asleep or awake, at the same clock times on consecutive days. Researchers introduced it in a 2017 study of 61 university students who kept sleep diaries for a month. Their index gives a schedule repeated exactly every day 100 and an entirely unpredictable one 0. The less regular students tended to have later body-clock timing and lower academic performance, an association rather than proof of cause.

A plain example: sleep from 11pm to 7am on Monday and again on Tuesday, and every hour lines up. If Tuesday runs from 1am to 9am instead, the hours from 11pm to 1am and from 7am to 9am no longer match. Regularity falls, even though both nights lasted eight hours.

What research suggests

In a UK Biobank study published in the journal Sleep, researchers calculated SRI from wrist-worn accelerometers for almost 61,000 adults and followed them for about six years on average. After adjusting for demographic, lifestyle and health factors, people with more regular sleep had a lower risk of death from any cause than the least regular fifth of participants. In their models, regularity predicted mortality more strongly than duration.

The study is observational, so it cannot show that changing your schedule would change your risk. Irregular sleep can reflect shift work, illness or caring responsibilities that also affect health. For everyday habits, the National Heart, Lung, and Blood Institute recommends regular bed and wake times, with weekends within about an hour of weekdays.

How My Bio Age calculates SRI

My Bio Age reads the sleep records in Apple Health, which often come from Apple Watch sleep tracking. It divides the 24-hour clock into 48 blocks of 30 minutes, marks the blocks between when you first fell asleep and when you last woke each night, and compares each block with the same block on the next recorded night, using up to 14 nights from roughly the last two weeks. Time awake during the night still counts as asleep, and a nap logged in Apple Health can stretch a night's span.

• The scale runs from 0 to 100. A score of 100 means your timing was identical from night to night; the more blocks that change, the lower it goes.
• It needs at least two nights, since there is otherwise nothing to compare.
• Coverage affects confidence. Five or more recorded nights a week gives high confidence, and at least three gives moderate confidence. Nights with no record at all are skipped rather than scored as zero, so they lower confidence instead of counting against the score. If your watch battery runs out partway through the night, though, the shorter night is still counted and does pull SRI down.

Because the app counts matching half-hour blocks, its figure is a simplified version of the research measure. It also reports the plain share of blocks that match, without the rescaling used in research, so a 50 in the app is roughly a 0 on the research scale and even a very erratic schedule rarely scores much lower. Its numbers therefore sit higher than SRI values published in studies, so use it to follow your own trend rather than to compare yourself with them.

How SRI is used

• A weekly Index input. SRI is one of the 12 core metrics behind the MyBioAge Index™. Recent complete weeks use that week's average SRI, and it counts for less in weeks with sparse sleep records. See how Apple Health metrics feed the Index.
• A critical checklist item. “Sleep + SRI” is one of three critical inputs on the Index data checklist, with “VO₂” and “Overnight HRV”. With fewer than about four recorded nights a week, the app marks the Index as limited and holds back week-to-week changes until coverage improves.
• A dashboard card. The Sleep Regularity card in the Overview section shows your recent trend, and its detail view charts your history.
• An AI Coach topic. You can choose Sleep Regularity when asking the on-device coach about a metric.

The detail view suggests aiming for an SRI of 80 or more, with at least five consistent nights a week. That is the app's rule of thumb for steadier timing, not a clinical target.

SRI and Circadian Alignment are different

Circadian Alignment is a separate daily 0–100 wellness score. It looks across up to 30 days and needs at least seven nights of sleep data. It combines its own regularity measure, based on how much your sleep midpoint varies, with the shift between weekday and weekend midpoints, the timing of heart-rate and HRV signals, and how often substantial activity falls shortly before sleep.

SRI feeds the weekly MyBioAge Index™; Circadian Alignment does not. The Circadian Alignment guide explains how that score is built.

To see how regular your own nights have been, download My Bio Age free on the App Store and connect your Apple Health sleep records. The iPhone app is completely free, with no in-app purchases, subscription or account required. Sleep calculations stay private on your iPhone, and optional AI Coach explanations run fully on-device.

Practical ways to steady your timing

For a practical weekly routine, see our guide to tracking sleep consistency with Apple Watch, with a simple weekly review of your bedtimes, and a guide to sleep habits for HRV and resting heart rate.

• Anchor your wake time. Pick a wake-up time you can keep most days, and plan bedtime back from the sleep you need.
• Keep weekends close. Stay within about an hour of your usual schedule on days off, as NHLBI suggests.
• Wind down. The same guidance recommends a quiet hour before bed, away from intense exercise and bright screens.
• Protect the record. Charge your watch before bed. Apple explains how to track sleep with Apple Watch.

Timing is only half the picture, so pair it with our sleep duration guide.

Shift work, a new baby or frequent travel can make a fixed schedule unrealistic. NHLBI has separate tips for shift workers, such as blocking light and noise during daytime sleep. A lower SRI then reflects your circumstances, not a failing.

My Bio Age calculates SRI on your iPhone from the sleep data you choose to share, as general wellness information rather than medical advice. If you regularly struggle to sleep, feel very sleepy during the day or are worried about your breathing at night, speak to a doctor or another qualified clinician.