In one sentence
How long you were actually asleep last night, which is the time in bed minus the time awake, and the one sleep number with the strongest evidence behind it.
Why it matters
Of everything a sleep tracker reports, duration is the measure the outcome evidence is built on. The American Academy of Sleep Medicine and the Sleep Research Society's consensus is seven hours or more a night for adults on a regular basis (Watson 2015). In 461,347 UK Biobank adults, habitual short sleepers (under six hours) had 20 percent more first heart attacks over the follow-up than people sleeping six to nine hours, long sleepers (over nine hours) 34 percent more, and a Mendelian-randomization analysis supported short sleep as a cause rather than a marker (Daghlas 2019); long sleep could not be tested the same way, which is why most of the field reads its association as the result of being unwell rather than the cause. In laboratory recordings of 5,273 healthy adults, total sleep averaged 395 minutes and fell by about ten minutes per decade of age (Boulos 2019).
What moves it
Your schedule, first: the time you give it. Then alcohol, which shortens the time to fall asleep and breaks up the second half of the night (Ebrahim 2013); caffeine late; a hot room; a late large meal; an early alarm; children; travel. Naps add to the day's total and VT shows them; the sleep scores leave them out, and Capacity counts them.
How to measure it well
The same device every night, worn snugly; or, with no device, the bedtime and wake time typed in, which gives a time-in-bed figure. A device is good at this number: in 34 healthy young adults tested against polysomnography, seven consumer devices scored sleep with a sensitivity of 0.93 or better, and most matched or beat research actigraphy (Chinoy 2021). The error runs one way: a device sees lying still as sleep, so it reads a restless night as longer than it was.
How Vitality Tracker uses it
Four ways. Duration carries 30 percent of the Sleep Health Index, scored against age-banded reference ranges (7 to 9 hours for adults, 7 to 8 over 65), with long sleep never penalized; it carries the most weight in the nightly sleep score; Capacity reads it against your own usual on the recovery side; and the last seven days of sleep are 40 percent of the lifestyle area of the Vitality Score. Sleep from a different device than usual is kept but does not enter the Sleep Health Index until that device has nights of its own.
How far to trust the number
Measured, to within tens of minutes on a typical night and worse on a broken one, because the wake inside the night is where devices err (see Time awake during the night). A seven-night average is a reliable picture of how much you sleep; a single night's figure can be twenty or thirty minutes optimistic.
Sources
- Watson NF, et al. Joint consensus statement of the American Academy of Sleep Medicine and Sleep Research Society on the recommended amount of sleep for a healthy adult: methodology and discussion. J Clin Sleep Med 2015;11:931–52. doi:10.5664/jcsm.4950
- Daghlas I, et al. Sleep duration and myocardial infarction. J Am Coll Cardiol 2019;74:1304–14. doi:10.1016/j.jacc.2019.07.022
- Boulos MI, et al. Normal polysomnography parameters in healthy adults: a systematic review and meta-analysis. 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
- Ebrahim IO, Shapiro CM, Williams AJ, Fenwick PB. Alcohol and sleep I: effects on normal sleep. Alcohol Clin Exp Res 2013;37:539–49. doi:10.1111/acer.12006
- Vitality Tracker Sleep Scoring Methodology, open specification v2.0.1, public edition. doi:10.5281/zenodo.23223863.