In one sentence
The sunrise and sunset for where you are, worked out from your location and the date, and the morning daylight you log against them, which VT reads beside your sleep timing because light is the strongest thing that sets your body clock.
Why it matters
The clock is set by light, and modern light is the wrong way round. When adults spent a week camping with only sunlight and firelight, their exposure to daylight rose, their light after sunset fell, and the internal clock advanced by about two hours, so that the biological night began at sunset and ended before waking just after sunrise; in ordinary life the same people had been seeing less daylight and more light after dark, and their clocks ran late (Wright 2013). In 88,905 UK Biobank adults who wore a light sensor for a week, with 3,750 deaths over eight years, brighter daytime light was associated with 16 to 34 percent lower all-cause mortality in the brightest tenth against the darker half, and brighter light at night with 21 to 34 percent higher, after adjustment for age, sex, season, and the usual lifestyle and social factors; cardiometabolic mortality carried the larger associations (Windred 2024). In the regularity study, the students with irregular sleep lived in light whose daily rhythm had about half the amplitude of the regular sleepers', and the modeling pointed to their light pattern as the main cause of their later clocks (Phillips 2017); in the chronotype study, more self-reported time outdoors in daylight advanced sleep for every chronotype (Roenneberg 2003).
What moves it
Sunrise and sunset move with the season and your latitude; in Auckland the sun rises at about 7:30 in June and 6:00 in December and sets at about 5:15 and 8:40, so the same routine sits in very different light across the year. The light you log is yours: outdoor daylight is tens of thousands of lux even under cloud, a bright room a few hundred, and the minutes outside in the first hours of the day are the part of the dose that sets the clock earliest.
How to measure it well
Keep your location current, because the sunrise is computed from it. Log morning daylight as a protocol: the Morning Sunlight protocol is built in, as is Evening Sunlight for the last light of the day; each takes a time and a duration like any other, and the Dawn and Sunset Alarms log ten minutes when you confirm. An Apple Watch writes a daily Time in Daylight figure, estimated from its light sensor, to Apple Health, and VT reads it as the device's estimate, kept apart from what you log. Light at night cannot be logged in minutes that mean anything; an evening wind-down protocol stands in for it.
How Vitality Tracker uses it
Sunrise and sunset are drawn on the day's timeline and on the sleep chart, so your bedtime, wake time and logged light can be read against the solar day; the three are shown together, with your chronotype, and nothing is scored. When no chronotype exists, the clock-time context the alignment component shows is this picture. Eir reads the light log when you ask about sleep timing or a late chronotype, and the Trajectory Layer will show how the seasons move your sleep timing against the sun.
How far to trust the number
The sunrise and sunset are exact to the minute for the location given. The light log is self-reported minutes, not lux; a watch's daylight minutes are an estimate from a sensor under a sleeve. Supporting by design: it explains the timing measures, it does not replace them.
Sources
- Wright KP, et al. Entrainment of the human circadian clock to the natural light-dark cycle. Curr Biol 2013;23:1554–8. doi:10.1016/j.cub.2013.06.039
- Windred DP, et al. Brighter nights and darker days predict higher mortality risk: a prospective analysis of personal light exposure in >88,000 individuals. Proc Natl Acad Sci USA 2024;121:e2405924121. doi:10.1073/pnas.2405924121
- Phillips AJK, et al. Sci Rep 2017;7:3216. doi:10.1038/s41598-017-03171-4
- Roenneberg T, Wirz-Justice A, Merrow M. J Biol Rhythms 2003;18:80–90. doi:10.1177/0748730402239679.