In one sentence
When you ate today, from your first meal to your last, taken from the fasting timer or the times you type on the web, and the overnight fast that runs from yesterday's last meal to today's first; the window's length, and more than its length its consistency, are circadian measures in their own right.
Why it matters
When healthy adults with no shift work photographed everything they ate for several days with a phone app, most ate frequently and erratically across their waking hours, the overnight fast simply tracked time in bed, under a quarter of calories were eaten before noon and more than a third after six in the evening, and a weekday-to-weekend shift in eating times, which the authors called metabolic jet lag, was common; for half the group the daily eating window exceeded 14.75 hours. When a small group of overweight participants with windows over 14 hours ate within 10 to 11 hours for 16 weeks, helped by a chart of their own eating pattern, they lost weight, reported more energy and slept better, and the benefits were still there a year later (Gill 2015). The hour of the last meal matters on its own: in a randomized crossover trial, 20 healthy volunteers ate the same dinner at 22:00 or at 18:00 and slept 23:00 to 07:00 in both arms; the late dinner pushed the post-meal period into sleep and, beyond that shift, produced higher overnight glucose, a delayed triglyceride peak, lower fat oxidation and higher cortisol, most markedly in the habitual earlier sleepers, while sleep architecture did not change (Gu 2020). The morning readings you wake to are partly last night's dinner time.
What moves it
Your day: the hour you leave work, the evening meal, the snack after it (a snack is a meal to the timer), the glass of wine with calories in it, and the weekend, when the whole pattern slides later. The first meal's time moves with your wake time and your chronotype. Travel and shift work rewrite it; illness and a diet break are marked and explained rather than counted.
How to measure it well
Use the fasting timer whether or not you fast: tap when you finish your last meal and tap when you break the fast, and the window and the fast follow. Logging meals with their times does the same without the taps. Water, black coffee and plain tea do not end a fast; anything with calories does. The forgotten tap is the one error that matters, a 20-hour fast that was really a late supper, and correcting the time on the fasting screen fixes it. The how-to, Use the fasting timer even if you don't fast, has the detail.
How Vitality Tracker uses it
Stored as the first and last meal times and the fasting duration, day by day. The overnight comparison reads a late or a large late meal as one of the things that explain an unusual night, so a late dinner is explained rather than counted against you. The day's window is drawn on the timeline, and the Nutrition page shows your average first and last meal times and how regular they are, the share of days each fell within 30 minutes of its average; that consistency is a circadian lever of its own. Eir reads the window against your glucose and your overnight readings when you ask why a morning looked the way it did. Time-restricted eating is one use of the timer; the record is the reason for it.
How far to trust the number
Self-reported, and only as good as the taps. The times are exact when they are entered; the omissions are the error. Read the week's pattern, not the day's, and treat a fast much longer than your usual as a missing tap until you have checked.
Sources
- Gill S, Panda S. A smartphone app reveals erratic diurnal eating patterns in humans that can be modulated for health benefits. Cell Metab 2015;22:789–98. doi:10.1016/j.cmet.2015.09.005
- 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, the engine catalogue of 15 October 2026 (meal timing and fasting; the overnight comparison's confounders).