In one sentence
The share of your time in bed that you spent asleep, which is the standard summary of how unbroken a night was.
Why it matters
Efficiency is the laboratory's single number for sleep continuity, and continuity is the part of sleep that changes with age and with being unwell. In the pooled records of 5,273 healthy adults, efficiency averaged 85.7 percent and fell by 2.1 points per decade (Boulos 2019). The consensus indicator of good sleep, 85 percent or more, is an expert-panel figure rather than a trial result, but it describes what people experience as a bad night. It has no outcome evidence of its own comparable to duration or regularity, which is why VT gives continuity as a whole 15 percent of the Sleep Health Index and duration twice that.
What moves it
Downward: anything that wakes you or keeps you lying awake: alcohol in the second half of the night, a late meal, caffeine, pain, a hot room, noise, a child, a full bladder, worry, and age across populations. Upward, falsely: a device that cannot see you lying still awake. Upward, truly: a regular schedule and a bedroom that is dark, quiet and cool.
How to measure it well
The same device every night, and the Night Log for the wake the device misses: a toilet trip or twenty minutes lying awake that you log is drawn over the device's chart, so you can see whether it caught it. At launch the log does not change the number.
How Vitality Tracker uses it
One third of the continuity component of the Sleep Health Index, with time to fall asleep and time awake, scored against age-banded laboratory norms after the device's known bias has been applied, or, for a device without a published validation, against your own 90-day baseline with the norms shown as context only. It is also a component of the nightly sleep score, a secondary reading in Symptom Radar (weight 0.7, lower is the direction it watches).
How far to trust the number
Estimate. Consumer wearables overestimate efficiency by 2 to 10 points, by an amount that differs by maker and firmware, because their wake specificity is low: 0.18 to 0.54 across seven devices in Chinoy 2021, 29 to 52 percent across six in the 2025 laboratory comparison the sleep specification cites. On the same device the trend is honest; the level flatters you.
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>)
- Chinoy ED, et al. Performance of seven consumer sleep-tracking devices compared with polysomnography. Sleep 2021;44:zsaa291. doi:10.1093/sleep/zsaa291
- Vitality Tracker Sleep Scoring Methodology, open specification v2.0.1, public edition (device-bias figures and the 2025 six-device comparison). doi:10.5281/zenodo.23223863.