In one sentence
How long it took you to fall asleep after getting into bed, a number that your own memory of the night measures about as well as a device does.
Why it matters
It is one of the three parts of sleep continuity and the one people notice most. In healthy adults it barely changes with age in laboratory records (about a minute per decade; Boulos 2019), so a drift upward in your own series is more likely to be about your evenings than your years. The consensus indicator, falling asleep within about 15 to 20 minutes, is expert opinion, held loosely: a few minutes either side means nothing, and a very short latency can be a sign of short sleep as easily as of good sleep.
What moves it
Upward: caffeine late, a bright screen, a late hard session, a big late meal, worry, a hot room, lying down before you are sleepy, and an irregular schedule, which leaves the body clock unsure when night is. Downward: alcohol, which shortens it at every dose and then spoils the second half of the night (Ebrahim 2013), and sleep debt.
How to measure it well
A device marks sleep onset when you stop moving, which is usually a little before you are asleep; a wrist is a poor judge of lying still awake. The sleep specification treats the sleep diary as arguably the better instrument for this one number, so a Night Log entry for "lying awake" at the start of the night is worth making when the device's figure looks wrong: it shows on the chart beside the device's onset, though at launch it does not change the number.
How Vitality Tracker uses it
One third of the continuity component of the Sleep Health Index, scored on the same tiers as efficiency, with diary entries scored against diary-appropriate anchors; and a component of the nightly sleep score, against a fixed target of 13 minutes. It is not a Symptom Radar reading.
How far to trust the number
Estimate. Device figures for onset latency were not different from the laboratory's on average in the ring study (de Zambotti 2019), and wake specificity across devices is low (Chinoy 2021), so the device can miss a long stretch of lying still. A typed estimate is at least as good.
Sources
- Boulos MI, et al. Normal polysomnography parameters in healthy adults. Lancet Respir Med 2019;7:533–43. doi:10.1016/S2213-2600(19)30057-830057-8>)
- Ebrahim IO, et al. Alcohol and sleep I. Alcohol Clin Exp Res 2013;37:539–49. doi:10.1111/acer.12006
- de Zambotti M, et al. The Sleep of the Ring. Behav Sleep Med 2019;17:124–36. doi:10.1080/15402002.2017.1300587
- Chinoy ED, et al. Sleep 2021;44:zsaa291. doi:10.1093/sleep/zsaa291
- Vitality Tracker Sleep Scoring Methodology v2.0.1. doi:10.5281/zenodo.23223863.