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…