Sleep Calculator
Work out the best times to sleep or wake based on sleep cycles, with the cycle timing explained.
Waking between cycles, not mid-cycle
Sleep runs in cycles, and waking in the middle of one tends to leave you groggy, while waking at the end of a cycle feels far more refreshing. This works out good times to go to sleep or wake up so that your alarm falls between cycles rather than in the depths of one.
How sleep cycles work
A full sleep cycle lasts roughly ninety minutes, moving from light sleep into deep sleep and then dreaming sleep before returning toward light sleep. You feel most rested waking during the light phase at the end of a cycle, and most groggy waking from deep sleep mid-cycle. Counting backward or forward in cycles from a fixed point gives times likely to fall at a cycle boundary:
| Cycles | Approx. sleep length |
|---|---|
| 4 cycles | about 6 hours |
| 5 cycles | about 7.5 hours |
| 6 cycles | about 9 hours |
Allowing a little time to fall asleep, then aiming for a whole number of cycles, is the idea behind waking refreshed.
A useful guide, not a precise science
It is worth being honest that this is an approximation. The ninety-minute cycle is an average, and real cycle length varies between people and even between nights, so the calculated times are a helpful guide rather than a guarantee — you might still wake mid-cycle sometimes. What matters at least as much is getting enough total sleep and keeping a consistent schedule, which does more for how rested you feel than timing alarms to cycles. Most adults need a certain range of total sleep, and persistent tiredness despite enough sleep is worth raising with a doctor. Used as a nudge toward both adequate sleep and better wake timing, though, thinking in cycles is a genuinely helpful habit.
Questions & answers
How long is a sleep cycle?
Roughly ninety minutes on average, moving through light, deep and dreaming sleep — though real cycle length varies between people and nights.
Why does waking between cycles feel better?
Because you wake from light sleep rather than deep sleep — waking mid-cycle from deep sleep tends to leave you groggy.
Is the cycle timing exact?
No — it is an average, so the times are a helpful guide, not a guarantee. Enough total sleep and a consistent schedule matter at least as much.
Is anything uploaded?
No. The calculation happens in your browser.