In one sentence
The average of every reading your sensor took in the period, and of any finger-pricks you log, a day, a week or a month, which is the simplest summary of your glucose and the one the Vitality Score reads.
Why it matters
The mean is the figure that the long-run measures are built on: the glucose management indicator is a straight line from it, and the laboratory's HbA1c is, in effect, a three-month version of it written into your red blood cells. In 153 healthy people wearing research sensors, the mean sat at 5.4 to 5.5 mmol/L (98 to 99 mg/dL), and 5.8 in the over-sixties (Shah 2019). It is also the figure that hides the most: two people with the same mean can have a flat day and a day of peaks and troughs, which is why the variability and meal entries exist.
What moves it
What you eat and when, most of all; then sleep, stress, being unwell, alcohol and training. The sleep effect is one of the clearest in the record: six nights of four hours in bed lowered glucose tolerance in 11 young men to a degree the authors compared with normal aging (Spiegel 1999), and a single night of four hours' sleep cut the glucose that nine healthy adults could dispose of under a clamp by about a quarter the next day (Donga 2010). A dinner at ten instead of six raised overnight glucose in 20 healthy volunteers, most in the early sleepers (Gu 2020). Alcohol lowers the overnight figure, and a fever raises it for days.
How to measure it well
Wear the sensor continuously and keep the coverage high: a day with half its readings missing has a mean of half a day. Compare means across periods of the same length on the same sensor site, and read the month's mean rather than the day's.
How Vitality Tracker uses it
The day's average glucose carries 30 percent of the glucose area of the Vitality Score, scored on the period's average, or, failing that, the last week's, or a single day up to 14 days old; glucose from sick days and the three days after is not carried forward. The month's mean is the basis of the glucose management indicator, and the Baseline Month's mean is the reference every later month is compared with. VT shows the healthy-adult reference beside it, labeled as the study's.
How far to trust the number
Measured, by a sensor whose readings sit within about 9 percent of the laboratory on average (the GS3's manufacturer figure), so the mean of a full day is reliable to a few tenths of a mmol/L and the mean of a month more so; a change of sensor site can shift it by more than that, which is why VT keeps the site with every reading.
Sources
- Shah VN, et al. Continuous glucose monitoring profiles in healthy nondiabetic participants. J Clin Endocrinol Metab 2019;104:4356–64. doi:10.1210/jc.2018-02763
- Spiegel K, Leproult R, Van Cauter E. Impact of sleep debt on metabolic and endocrine function. Lancet 1999;354:1435–9. doi:10.1016/S0140-6736(99)01376-801376-8>)
- Donga E, et al. A single night of partial sleep deprivation induces insulin resistance in multiple metabolic pathways in healthy subjects. J Clin Endocrinol Metab 2010;95:2963–8. doi:10.1210/jc.2009-2430
- Gu C, et al. Metabolic effects of late dinner in healthy volunteers: a randomized crossover clinical trial. J Clin Endocrinol Metab 2020;105:2789–802. doi:10.1210/clinem/dgaa354
- Vitality Tracker, What VT measures (catalogue of 15 October 2026).