Sleep Regularity Index (SRI)
How consistent your sleep timing is: the dimension duration misses
Plain English
The Sleep Regularity Index (SRI) is a measure from 0 to 100 of how consistent your sleep and wake timing are from one day to the next. An SRI of 100 means you sleep and wake at exactly the same time every day. An SRI around 85 reflects solid real-world consistency. Large population research has found that sleep regularity predicts metabolic health and mortality risk independently of sleep duration, capturing something meaningful that hours-per-night counting alone does not.
The Mechanism
The SRI was developed by sleep researchers at Oxford and later extensively studied by Dr. Andrew Phillips and colleagues at Brigham and Women's Hospital (Harvard), who analyzed wrist-worn actigraphy data from more than 60,000 participants in the UK Biobank. The index calculates the probability that you are asleep or awake at any two time points exactly 24 hours apart, averaged across many days. Sleeping at 11 PM one night and 2 AM the next lowers the SRI because consecutive days look different to the biological clock.
The biological mechanism behind SRI's health effects runs through the circadian system. Inconsistent sleep timing sends conflicting signals to the suprachiasmatic nucleus (SCN), the brain's master clock. When the SCN cannot establish a stable phase because wake and sleep times vary daily, it cannot send coherent timing signals to peripheral organ clocks in the liver, gut, and muscle. These organs operate on their own rhythms, and when they drift out of sync with the SCN, metabolic processes degrade: insulin sensitivity is worse when eating at a time the liver is not prepared for, inflammation markers rise, and glucose regulation becomes less precise.
A 2023 study by Phillips and colleagues published in Sleep, using UK Biobank data, found that participants with an SRI below 72 had significantly higher all-cause mortality risk compared to those with SRI above 87, after controlling for total sleep duration, sleep quality, physical activity, BMI, and other confounders. Duration and regularity appear to measure partially overlapping but distinct dimensions of sleep health.
Why It Matters
Your biological clock cares more about consistency than duration.
Sleep research has spent decades focused on duration: get 7-9 hours. The SRI evidence adds a second dimension: when you sleep, and whether that timing is consistent, is an independent variable with real health consequences. You can average 7.5 hours per night and still have poor SRI if bedtime varies by 2+ hours between weekdays and weekends. That variability alone is associated with measurable metabolic and cardiovascular risk. A consistent 7-hour sleeper likely has better long-term metabolic outcomes than an irregular 8-hour sleeper.
Common Misconception
Most sleep advice focuses on total hours. The SRI research suggests that a consistent 6.5-hour sleeper with high regularity may have better long-term health outcomes than someone who averages 8 hours with 2+ hours of timing variability between weekdays and weekends. Regularity does not replace adequate duration, but it is an independent dimension of sleep health that hours-counting entirely misses.
What a Healthy Range Looks Like
Low
SRI <72
Associated with significantly elevated all-cause mortality risk in large population studies (Phillips et al., 2023)
Moderate
SRI 72–85
Some timing inconsistency; typical of people with variable weekend schedules
Good
SRI 85–95
Solid real-world consistency; associated with better metabolic and cardiovascular markers
Excellent
SRI 95–100
Highly consistent sleep timing; achieved by disciplined or naturally regular sleepers
SRI above 87 is associated with meaningfully lower mortality risk in the UK Biobank data (Phillips et al., Sleep, 2023). Real-world good performance sits around 85-95. Occasional irregularity from travel or illness does not meaningfully shift a strong average. Compare your trend across weeks, not individual nights.
Signs It Is Disrupted
- Sleep and wake times that vary by 2 or more hours between weekdays and weekends, a classic social jetlag pattern.
- Difficulty falling asleep at a consistent time each night even when genuinely tired.
- Morning grogginess that varies significantly by day of the week rather than by sleep duration.
- Energy crashes or appetite irregularity that tracks with shifts in sleep timing rather than total hours.
How to Improve It
Which Devices Track It
Oura Ring
Oura reports a sleep regularity metric based on consistency of sleep and wake timing across days. The underlying calculation is similar in spirit to the SRI, though the exact formula differs from the research version. Trend data across two or more weeks is more meaningful than single-week readings.
WHOOP
WHOOP tracks sleep consistency as part of its weekly performance reports and flags irregular sleep timing, but does not report a discrete SRI-equivalent numerical score.
Apple Watch / Health App
Apple Health tracks sleep schedule consistency and surfaces it in sleep summaries. The visualization shows time-in-bed distribution across the week rather than a discrete regularity score.
3 Things to Remember
The Sleep Regularity Index measures timing consistency independent of duration: UK Biobank research (Phillips et al., 2023) shows SRI below 72 is associated with significantly elevated mortality risk regardless of how many hours are slept.
Sleep regularity and sleep duration are distinct dimensions: a consistent 7-hour sleeper may have better metabolic outcomes than an irregular 8-hour sleeper because consistency determines how well the circadian system can coordinate organ function.
A consistent wake time is the highest-leverage intervention for improving SRI: sleep timing generally stabilizes once the morning anchor is fixed.
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