Sleep calculator
Waking up mid-cycle is why mornings hurt
Tell it when you need to be up and it counts backwards in whole sleep cycles, so the alarm lands where sleep is lightest instead of halfway through the deepest part. It works the other way round too.
Go to bed at one of these
Each one lands you at the end of a cycle rather than in the middle of one.
- 9:45 pm6 cycles, 9h of sleepasleep by 10:00 pm
- 11:15 pm5 cycles, 7h 30m of sleepasleep by 11:30 pm
- 12:45 am4 cycles, 6h of sleepasleep by 1:00 am
Cycles are not a metronome. Yours will run somewhere around 70 to 120 minutes, the early ones are heavier on deep sleep and the late ones on REM, and the length drifts across the night. Treat these as good places to aim, not as a schedule you are failing.
The sum, in full
7:00 am minus (5 x 90 min) minus 15 min = 11:15 pm
That is the whole model: 90 minutes a cycle, plus the time you told us it takes you to drop off. Most adults do well on 7 to 9 hours, which is why the five cycle row is the one highlighted.
What the number is doing
Ninety minutes is an average, and averages have edges
A night is not one long slide into unconsciousness. You go down through light sleep into deep sleep, come back up into REM, and surface briefly before starting again. One trip round takes most people somewhere around 90 minutes, so a calculator that counts in 90 minute blocks is aiming your alarm at the shallow end of a lap.
Where it gets loose is that nobody's cycle is exactly 90 minutes, and yours is not the same length all night. The first two laps carry most of your deep sleep. The last two are heavy on REM and tend to run longer. Add the fact that falling asleep took some amount of time you did not measure, and by the fifth cycle a 90 minute model can easily be half an hour out.
That does not make it useless. Aiming at the right half hour beats aiming at nothing, and the discipline of picking a bedtime and keeping it is worth more than the precision of the number. It does mean the times above are a target rather than an instruction.
The part a calculator cannot do
Every one of these tools is doing arithmetic on a guess about you. It does not know when you actually fell asleep, how long your cycles ran last night, or that you surfaced three times because the room was too warm. The only way to aim an alarm at a genuinely light moment is to watch the night while it happens.
That is what Tock does. It listens and feels for movement in the last half hour before your alarm and rings when you are already close to the surface, and your set time is always the fallback, so it can only ever be early and never late.
Questions
What people ask about bedtime maths
What time should I go to bed if I wake up at 7am?
Counting back in ninety minute cycles from 7am, and allowing about fifteen minutes to fall asleep, gives 9:45pm for six cycles, 11:15pm for five and 12:45am for four. Five cycles is 7 hours 30 minutes of sleep, which sits inside the 7 to 9 hours most adults are advised to get.
How long is a sleep cycle really?
Ninety minutes is an average, not a constant. A real cycle runs somewhere around 70 to 120 minutes, varies between people, and changes across the night: the early ones carry more deep sleep and the later ones more REM. That is why a cycle calculator gives you a target rather than a guarantee.
Why do I feel worse after eight hours than after six?
Because it is not only how long you slept, it is where in a cycle the alarm caught you. Being pulled out of deep sleep produces sleep inertia, the heavy, slow feeling that can take fifteen to thirty minutes to clear. Waking near the light end of a cycle usually feels better even if the night was shorter.
Does the calculator account for how long I take to fall asleep?
Yes. Pick the option that matches you and every bedtime shifts by that amount. Fifteen to twenty minutes is typical for an adult. If you regularly lie awake for more than thirty minutes, that is worth looking at on its own rather than solving with an earlier bedtime.
Is a sleep cycle calculator accurate?
It is accurate arithmetic on an average. It cannot know your cycle length, how long you actually took to drop off, or how many times you surfaced during the night, because none of that has been measured. Treat the times as good places to aim rather than as a schedule you are failing.
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