How pre-flight shift calculator Works
The 1-hour shift protocol
The circadian system shifts about 1 hour per day when actively driven (sleep schedule + light timing). Pre-flight shifting moves bedtime and wake time 1 hour daily toward the destination — eastward earlier, westward later — for as many days as the calendar allows. Four disciplined days pre-cross four zones.
Light makes the shift stick
The bedtime move alone drifts; the light schedule drives. Eastward shifts pair morning-bright on waking; westward shifts pair evening brightness. Without the light half, the schedule half mostly produces tired people going to bed at strange hours — the light is what tells the clock the new time is real.
When it is worth the discipline
Pre-shifting pays past ~5 zones and past 3+ day trips — the recovery math halves, and the first two days of the trip become usable. For short hops and westward vacations, arrival-mode (light schedule + local meals) is simpler and nearly as effective. Match the tool to the stakes.