Sleep Cycle and Jet Lag Calculator
Two bits of arithmetic people do badly on the back of an envelope. Counting cycles back from an alarm is easy until you forget the time it takes to fall asleep — which is the whole reason the answer felt wrong. And a jet lag plan goes astray because clocks wrap at midnight, and because shifting the wrong way round the circle turns three hours into nine.
What this generator does
Subtracts whole sleep cycles and your settling time from the alarm to give bedtimes, and divides a time-zone difference into daily steps that sum to it exactly.
How to use this tool
- Type the time your alarm is set for.
- Say how long you take to fall asleep — fifteen minutes is the usual figure.
- Pick a bedtime from the list; each lands at the end of a cycle.
- For a trip, say how many hours ahead the destination is and how fast you want to shift.
Understanding the controls
- Alarm set for
- The time you have to be up. Every bedtime is counted backwards from here, wrapping over midnight where it needs to.
- Minutes to fall asleep
- The gap between getting into bed and being asleep. Leaving it out is why hand-counted bedtimes come out too late by exactly that much.
- Hours the destination is ahead
- Positive going east, negative going west. The direction bedtime moves follows from the sign, so it cannot be set the wrong way round.
- Minutes to move each day
- How aggressively to shift. An hour a day is brisk; half an hour is gentle and takes twice as long.
Common use cases
- Working out when to go to bed for an early start
- Choosing between six cycles and five when you are already up late
- Planning the week before a flight so you land part way adjusted
- Seeing how many days a shift of five hours actually takes
- Understanding why waking mid-cycle feels worse than less sleep would
How this generator works
A cycle is taken as ninety minutes, which is the conventional figure. Bedtimes are the alarm less a whole number of cycles less the settling time, and the check replays each one forwards — bedtime plus settling plus sleep must land exactly on the alarm, wrapping at midnight. The jet lag plan divides the time-zone difference into daily steps, with the last day taking whatever is left rather than overshooting, and the check confirms the steps sum to exactly the shift, that no day moves more than the step chosen, and that every bedtime is still a real time of day after wrapping.
Randomness and fairness
Nothing is random. The same alarm and settling time always give the same bedtimes.
For how randomness is produced across the whole site, see how Generate Random works.
Limitations and good to know
- Ninety minutes is an average; real cycles vary between people and between nights, so treat the times as a guide.
- This is arithmetic about clocks, not advice about sleep or health.
- The jet lag plan shifts bedtime only; light, meals and caffeine matter as much and are not scheduled.
- Daylight saving changes during the shift are not accounted for.
Privacy and your data
The times are worked out in your browser. Nothing about your sleep or your travel is transmitted or stored.
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