In one sentence
VT's own 0-to-100 reading of last night, built from the things a device measures well, and labeled low-confidence because a single night is mostly noise.
Why it exists, and why it is labeled
People want to know how last night went, and VT answers in the open: the score weighs how long you slept against your own sleep need (duration counts most), efficiency, time awake after falling asleep and the number of wakings judged against your own usual, time to fall asleep against a fixed target, how closely you kept your usual schedule, and overnight recovery from your heart rate, HRV, breathing rate and blood oxygen. What it does not use is sleep stages. The label is the honest part. Almost all the outcome evidence concerns patterns over weeks, not nights; even in laboratory-grade home recordings, the deep-sleep share varies night to night by about a fifth of its mean and the latencies and wake time by half, so a night's figure carries limited information even when measured perfectly. The number you are asked to improve is the Sleep Health Index. This one tells you what happened.
What moves it
A short night, most of all; then a broken one; then an unusual schedule; then a night when the overnight readings sat outside your usual, which they do after alcohol, a late meal, a hard session or at the start of being unwell.
How to read it
As a line across the week, not a verdict on the night. A low score after an evening you logged is explained; a run of low scores with nothing logged is a question for the Sleep Health Index and, if it persists, for your doctor.
How Vitality Tracker uses it
Computed in the app each morning, from the first synced night, and shown as the night's Sleep Quality unless your sleep source scores the night itself, as Oura and Polar do. Capacity reads the overnight readings and the night's duration and regularity directly, and the Sleep Health Index is built from the 28-night record, not from nightly scores. With Apple Health or Health Connect as your sleep source, which give no score of their own, it also nudges the sleep part of the Vitality Score's lifestyle area. Continuity is judged against your own usual so that a device that always reports a lot of awake time is not penalized.
How far to trust the number
Calculated from measured and estimated inputs, so it inherits their accuracy; low-confidence by label and by design.
Sources
- Vitality Tracker, How Vitality Tracker scores sleep, method note v1.0 (8 October 2026)
- Vitality Tracker Sleep Scoring Methodology, open specification v2.0.1, public edition. doi:10.5281/zenodo.23223863.