Most jet lag apps prescribe a plan on day zero and forget about you. Shift is different. It uses your Apple Watch wrist temperature — the data your watch already collects every night — to estimate where your body clock actually is, and adapts the protocol every morning as your body responds.
Wrist temperature is a clinically supported proxy for core body temperature minimum (CBTmin) — a gold-standard circadian phase marker. Light cues follow the published Phase Response Curve. Personal adaptation speed is learned with a Bayesian update after every trip — eastward and westward are tracked separately because they aren't symmetric.
No accounts. No servers. No analytics. Shift reads HealthKit on device and never writes back. Phase estimation and protocol generation happen locally — your data never leaves the phone.
Shift requires Apple Watch Series 8 or newer — the first model that records wrist temperature. iPhone 12 or newer. iOS 17+ / watchOS 10+.