Sundial: Body Clock

Your body clock has a needle: the time your core temperature is lowest. Light before it delays the clock; light after it advances the clock. So "get morning sun" is right at home on your usual schedule, and backwards on the first morning in a new time zone or after a weekend sleep-in.

Sundial estimates where that needle is from your Apple Watch sleep and heart rate during sleep, and looks only at the gap to where you want it.

■ Three situations, one mechanism • Trips: shift your sleep step by step for a few days before departure; after arrival, get windows for seeking and avoiding light in local time. Eastward and westward are different jobs, and it knows. • Shift changes: from the last day of your old shift, use your days off to move gradually. Enter working hours and it can start on work days too. • Daily life: only on a morning your needle has drifted (say, after a weekend), you get a timetable to bring it back. On aligned days: "Today is free."

■ A five-line timetable Seek/avoid light, wake and bedtime, first and last bite, exercise window, bath. All computed from your needle for that day only.

■ No assumptions It doesn't assume you wake at 7. Your baseline is learned from your own last 14 wake times. Night owls and late shifts keep their own life as the baseline.

■ Your data stays on your device Apple Health data and everything computed from it stay on your iPhone. No server. No ads, no tracking. Every feature is free for individuals.

■ Evidence Needle = core temperature trough, 3–4.5 h before waking (Eastman & Burgess 2009). Light phase response (Khalsa 2003), exercise phase response (Youngstedt 2019), meals and peripheral clocks (Wehrens 2017), pre-flight adjustment (Burgess 2003). Sundial is not a medical device and does not promise to cure jet lag.

Apple Watch (sleep tracking) recommended. Without a Watch, enter your usual wake time and the timetable still works.

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