In one sentence
Chronotype is when your body prefers to sleep, measured by the Munich ChronoType Questionnaire as your corrected mid-sleep on free days; circadian alignment is how closely the timing you actually keep sits to it, the component of the Sleep Health Index that scores timing against you rather than against the clock.
Why it matters
The questionnaire was built from 500 pilot responses that recorded sleep times on work days and free days separately: most people slept at different times on the two kinds of day, late types accumulated a sleep debt during the work week and lengthened their free-day sleep by hours to repay it, and for every chronotype more self-reported time outdoors in daylight advanced sleep; the authors concluded that chronotype is genetic disposition, sleep debt and light together (Roenneberg 2003). Light's share was shown directly when adults spent a week camping with no electric light: the internal clock advanced by about two hours, the biological night came to begin at sunset and end before waking just after sunrise, and the later chronotypes advanced the most (Wright 2013). Alignment is scored rather than clock time because a timing judgment against a fixed window (an earlier version of VT's score used mid-sleep between 02:00 and 04:00) is a judgment against someone else's clock; the external reviewer observed that a member who had not declared a chronotype was receiving an absolute clock-time verdict that may be less valid than no verdict, and the component was rebuilt as alignment with an evidence hierarchy.
What moves it
Chronotype: age, later through adolescence and earlier through later life; genes; and light, morning daylight pulling it earlier and evening light pushing it later. Alignment: your schedule, the alarm, work, children, and the choices you make in the evening; social jet lag is what misalignment looks like across a week.
How to measure it well
Complete the questionnaire once in the app: it asks when you go to bed, how long you take to fall asleep, when you wake, whether an alarm wakes you, and how many days a week you work, separately for work days and free days; repeat it after a real change in your life, and VT asks again each year. The wearable supplies the other half, your habitual timing over 90 days, once you have 28 nights, which is also the fallback when the questionnaire has not been done.
How Vitality Tracker uses it
Circadian alignment is 10 percent of the Sleep Health Index and is scored by a strict hierarchy. With a completed questionnaire, or enough history to estimate your chronotype from behavior, it scores the gap between your habitual sleep timing and your own corrected mid-sleep. With no chronotype but at least 28 nights, it scores internal consistency, whether you are sleeping when you normally sleep, at moderate confidence. With neither, the component is suppressed and the index is computed over the components present with the coverage saying so; the association with clock time is then shown as weak context only, never scored. Chronotype itself is shown as a mid-sleep time with the band it falls in, and social jet lag is shown beside it.
How far to trust the number
The questionnaire is the instrument chronotype epidemiology is built on and is self-reported by design; the behavioral estimate from 28 nights is its objective cousin and agrees with it to within an hour for most people. The alignment component is graded C in the specification, weak or contested evidence, which is why it carries 10 percent and not more, and why social jet lag is kept out of it. Shift workers should expect the questionnaire's assumptions to fit poorly, and the app says so.
Sources
- Roenneberg T, Wirz-Justice A, Merrow M. Life between clocks: daily temporal patterns of human chronotypes. J Biol Rhythms 2003;18:80–90. doi:10.1177/0748730402239679
- 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
- Roenneberg T, et al. Curr Biol 2012;22:939–43. doi:10.1016/j.cub.2012.03.038
- Vitality Tracker Sleep Scoring Methodology v2.0.1 (section 5.5). doi:10.5281/zenodo.23223863.