Wearable metric
Sleep regularity index
SRI · Sleep consistency
How consistent your sleep–wake timing is from day to day. In a 60,000-person UK Biobank study it predicted mortality better than how long you slept.
What it measures
The Sleep Regularity Index scores, from 0 to 100, the probability that you are in the same state — asleep or awake — at any two points 24 hours apart. 100 is perfectly identical timing every day; 0 is fully random. It captures the consistency of your schedule, separate from how much sleep you get.
Reference context
1 guideline source
There is no clinical cut-off for the SRI — cohort data simply show higher-is-better across the range. A practical read: keep your sleep and wake times within roughly the same hour across the week, weekends included.
Population context — consult guideline targets below
Mechanism
Why moving this marker matters
The circadian system runs on regular light and behavioural cues. Irregular sleep–wake timing desynchronises peripheral clocks from the central pacemaker, a state linked to metabolic, cardiovascular and mood disruption. Regularity may matter because the body anticipates and prepares for predictable sleep and wake.
Guideline targets
What major guidelines recommend
Windred 2024 (UK Biobank cohort)
Higher SRI tracks lower mortality; the most regular sleepers had roughly 20–48% lower all-cause mortality than the most irregular
Observational association, not a treatment target or threshold.
How to measure
The test, where to get it, when to repeat
Method
Computed from continuous accelerometry or a sleep-tracking wearable over at least a week. Research apps report the SRI directly; consumer wearables increasingly surface a 'sleep consistency' analogue.
Where
Free with a sleep-tracking wearable or research-grade actigraphy.
Typical cost
Free with device.
Fasting
Not required
When to test
Sleep-health framing
Read as a rolling weekly trend, not a single night; the value of regularity is in sustained consistency.
How to track
Devices and apps that measure this
Most consumer wearables do not report this metric reliably yet. A clinician or sports-medicine lab can measure it directly.
Context
Reading the numbers
There is no clinical cut-off for the SRI — cohort data simply show higher-is-better across the range. A practical read: keep your sleep and wake times within roughly the same hour across the week, weekends included.
Caveats
Consumer-device 'consistency' scores are not the validated research SRI and are computed differently between brands — use the trend within one device, not absolute cross-brand comparisons. Shift workers will score low for reasons outside their control.
Practices
What's been shown to influence this marker
Fixing wake time and anchoring it with morning light is the most direct lever on regularity; consistency improves with a stable schedule.
Sleep regularity
Hit the same bedtime within a 30-minute envelope. Stronger mortality signal than total hours slept.
Why
Cohort studies of older adults consistently show that going to bed and waking at consistent times — within a roughly 30-minute window — predicts mortality risk independent of how many hours someone sleeps. The body's circadian system entrains to expected timing; irregularity creates a low-grade jet-lag effect day after day.
Slot in your day
How to do it
How
Pick a target bedtime. Hold it within ±30 minutes, including weekends, for 4 weeks. Pair with morning sunlight within an hour of waking.
Ideal for
Anyone with shifting work hours or weekend social rhythms.
Sticking with it
Pick the bedtime that's actually possible 6 nights a week, not the aspirational one.
Markers this may influence
Evidence
Windred 2024 Sleep (UK Biobank, n=60,977, mean 6.3-y follow-up): higher Sleep Regularity Index was associated with 20–48% lower all-cause mortality across the top four quintiles vs. the most irregular. Notably, sleep regularity outperformed total sleep duration as a mortality predictor.
Morning sunlight
10 minutes outdoors within an hour of waking anchors the circadian rhythm.
Why
Bright outdoor light in the first hour after waking suppresses melatonin and sets the body's internal clock for the day, making evening sleep onset easier. Cloudy days still deliver several thousand lux outdoors — far more than typical indoor lighting.
Slot in your day
How to do it
How
10–15 minutes outdoors, preferably without sunglasses, ideally walking. Light therapy lamps (10,000 lux) work as a substitute when outdoors isn't possible.
Sticking with it
Pair it with a fixed morning anchor — coffee, walk, school run. The pairing carries the habit.
Markers this may influence
Evidence
Wright 2013 Curr Biol (n=8): one week of camping with no electric light advanced melatonin onset by ~2 hours and aligned circadian phase with the natural light/dark cycle — small but mechanistic confirmation that morning bright light is the dominant entraining signal. Khalsa 2003 mapped the human phase-response curve to single bright-light pulses.
Afternoon caffeine cutoff
No caffeine after 2pm — half-life is 5–7 hours, longer in slow metabolisers.
Why
Caffeine blocks adenosine receptors that signal sleep pressure. With a half-life of 5–7 hours (longer in some genotypes), an afternoon coffee can leave a quarter of the dose still active at bedtime. Effect on sleep architecture (less deep sleep) is measurable even when subjective sleep feels fine.
Slot in your day
How to do it
How
Last caffeine by 2pm if you sleep around 11pm. Adjust earlier if you metabolise slowly (genetic variation in CYP1A2). Switch to decaf or herbal alternatives in the afternoon.
Sticking with it
Pre-stage a tasty afternoon non-caffeine drink. Removing without replacing fails.
Markers this may influence
Evidence
Drake 2013 J Clin Sleep Med (within-subject lab study): 400 mg caffeine taken 6 hours before bed still measurably disrupted polysomnography-assessed sleep — total sleep time fell by ~1 hour vs placebo, even when taken hours before bedtime. The 5–7 hour half-life puts a meaningful fraction of an afternoon dose into the sleep window.
See also
Related markers
Take to your physician
Worth discussing
- Whether shift work or a circadian rhythm disorder explains a persistently low score.
- How to anchor a regular schedule if work or caregiving makes it hard.
Sources
Cited literature
- [1]Windred et al., Sleep regularity is a stronger predictor of mortality risk than sleep duration — 60,000 UK Biobank participants (Sleep)(2024)
- [2]Phillips et al., Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing (Scientific Reports)(2017)
Edited by Carl Pöhl, MD · Healicus editorial
Last reviewed May 2026
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