Sleep calculator, caffeine cutoff and sleep debt

1. Pick the tab that matches your problem. Use the first one when the alarm is fixed and you want to know when to go to bed, the second when you are already tired and want a sensible alarm.
2. Check the latency figure. The default of 15 minutes is the middle of the normal range. If you know you lie awake for 30 minutes, put that in and every row shifts earlier.
3. Aim for the highlighted rows. Five or six cycles is 7.5 or 9 hours, which covers the range most adults need.
4. In the nap tab, choose a length before you lie down rather than after. Ten to twenty minutes for a pick-me-up, ninety if you have the time and are properly short of sleep.
5. Set your usual bedtime and your last caffeine dose in the cutoff card. The bars show roughly how much is left in you each hour, and the cutoff time tells you when to stop tomorrow.
6. Fill in the ledger each morning. After a few days the weekly deficit gives you a number that is harder to argue with than a feeling.

The 90-minute cycle is an average, not a metronome. Sleep runs in cycles of light sleep, deep sleep and REM, but the length varies between people and between cycles in the same night, from roughly 70 to 120 minutes. Early cycles carry more deep sleep and later ones carry more REM. No calculator can tell you where you are in that pattern without measuring you, so the times on this page are good guesses that get less reliable as the night goes on.

Total sleep beats cycle timing

If you have to choose between hitting a cycle boundary and getting more sleep, take the sleep. Sleeping 8 hours and waking mid-cycle beats sleeping 6 hours and waking neatly. The main use of these rows is to stop you setting an alarm that gives you five and a quarter hours when six was available.

Sleep need is a range

Adults generally need 7 to 9 hours. Some people genuinely function on less, but far fewer than the number who believe they do. Van Dongen and colleagues (2003) held people at six hours a night for two weeks. Their performance kept degrading until it matched two full nights without sleep, while their own ratings of sleepiness levelled off early. You cannot feel the deficit accurately, which is the whole reason a written ledger helps.

Debt does not fully repay

The weekly deficit here is arithmetic, not biology. Recovery sleep restores some function quickly, especially after one bad night, but studies of chronic restriction find that reaction time and attention do not return to baseline as fast as people expect. Treat a large deficit as a signal to change the schedule rather than a debt you can clear on Saturday.