Sleep cycle calculator
Count back from your wake time in 90 minute blocks, and treat the answer as a range rather than a target.
How this works
Sleep runs in cycles of roughly 90 minutes. Waking at the end of a cycle, in lighter sleep, generally feels better than being woken out of deep sleep.
The calculator counts back in 90 minute blocks and adds time to fall asleep.
What the calculator cannot know
Worth saying plainly, because these tools are frequently presented with more precision than they deserve.
Ninety minutes is an average. Real cycles run roughly 70 to 120 minutes, they vary between people, and they vary across the night for the same person.
Cycle composition changes as the night goes on. Deep sleep front-loads the night and REM back-loads it, so the later cycles are not like the earlier ones.
Falling asleep is not instant and not consistent. Ten to twenty minutes is normal, and it changes with how tired you are.
So treat the output as a sensible bedtime range rather than a target to hit. Being ten minutes out is not worth worrying about, and worrying about it will cost you more sleep than it saves.
What is happening in a cycle
| Stage | What it does |
|---|---|
| Light sleep (N1, N2) | The transition in and the bulk of the night. Easy to wake from. |
| Deep sleep (N3) | Physical restoration, growth hormone, factual memory. Front-loads the night. |
| REM | Most dreaming, emotional and procedural memory. Back-loads the night. |
Sleep inertia is why this matters at all: being woken out of deep sleep produces heavy grogginess that can last twenty to thirty minutes, where waking from light sleep does not.
Two things that beat timing your cycles
A consistent wake time, seven days a week. More effective than a consistent bedtime, and more effective than any calculation, because it aligns your body clock so the cycles fall where they should on their own.
Enough total sleep. Seven to nine hours for most adults. Five perfectly timed cycles is 7.5 hours, which is within range; four is 6 hours, which is not, however neatly it lands.
Use the calculator to pick between two plausible bedtimes. Do not use it to justify a short night on the grounds that it is a whole number of cycles.
If the problem is not timing
If you sleep a full night and still wake unrefreshed, that is usually fragmentation rather than cycle timing, and something is interrupting cycles before they complete. Temperature is the commonest and most fixable cause: 7 to 10.5 tog suits most modern UK bedrooms.
If falling or staying asleep has been the problem for weeks rather than nights, cognitive behavioural therapy for insomnia is the evidence-based first-line treatment in the UK and a GP can refer you.
And pauses in breathing during sleep, gasping awake, loud snoring or heavy daytime sleepiness need a GP, because sleep apnoea fragments sleep without waking you enough to remember it.
Related reading
How sleep works
Sleeping better
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