In one sentence
When your sleep started and ended, kept with the time zone you were in, which is the raw material of the sleep measure with the strongest outcome evidence of all: regularity.
Why it matters
The two clock times matter less for where they fall than for how much they move. The Sleep Regularity Index asks a simple question of 28 nights of data: at any two moments 24 hours apart, how likely were you to be in the same state, asleep or awake (Phillips 2017). In 60,977 UK Biobank adults with more than ten million hours of wrist recordings, the most regular fifth had 20 to 48 percent lower all-cause mortality than the least regular fifth across the adjustment models, and regularity predicted mortality better than duration did; adding duration to the regularity model did not improve it (Windred 2024). Behaviorally, the most regular fifth fall asleep and wake within about an hour's window every day, weekends included. Regularity is also the sleep behavior most within your control, which is why it carries the largest weight in VT's Sleep Health Index. Irregular timing drags the body clock with it: in 61 students, the irregular sleepers' melatonin onset came about two and a half hours later than the regular sleepers' (Phillips 2017).
What moves it
You, mostly: alarms, work, children, evenings out, screens, and the weekend. Travel moves it in clock time without moving it in body time, which is why VT keeps the zone. A late bedtime also moves the overnight readings for that night, and VT reads "late" against your own habit rather than the clock.
How to measure it well
A device gives both times without effort. With no device, type them in; with one, you can correct its times by hand when it gets them wrong. If you move time zones, let the phone's clock follow you.
How Vitality Tracker uses it
Three things come from these two times. Regularity, 35 percent of the Sleep Health Index, computed as the Sleep Regularity Index over the last 28 nights, with a multiplier that only reaches full strength when low regularity persists across two consecutive 28-night windows, so that one disrupted month is an episode and not a verdict. Time awake during the night, worked out from the window and total sleep. And a late-bedtime flag that Symptom Radar uses to give that night's readings less weight, judged against your own habits once VT has about 20 logged examples from the last 90 days. Social jet lag, the shift between weekday and weekend timing, is shown but not scored; alignment with your chronotype is the circadian component, scored only when you have completed the chronotype questionnaire (see the circadian entries).
How far to trust the number
Measured, to within minutes, by any device that can tell sleep from wake at the edges of the night; the uncertainty is at the moment of falling asleep, which is the subject of Time to fall asleep. Typed times are as good as your memory of them.
Sources
- Windred DP, Burns AC, Lane JM, Saxena R, Rutter MK, Cain SW, Phillips AJK. Sleep regularity is a stronger predictor of mortality risk than sleep duration: a prospective cohort study. Sleep 2024;47:zsad253. doi:10.1093/sleep/zsad253
- Phillips AJK, et al. Irregular sleep/wake patterns are associated with poorer academic performance and delayed circadian and sleep/wake timing. Sci Rep 2017;7:3216. doi:10.1038/s41598-017-03171-4
- Vitality Tracker Sleep Scoring Methodology, open specification v2.0.1, public edition. doi:10.5281/zenodo.23223863.