How to Use
- Choose mode
Select whether you want to calculate from your wake-up time or bedtime.
- Enter time
Input the time you need to wake up or plan to go to bed.
- View results
See optimal bedtime or wake-up options aligned with complete sleep cycles.
What is a sleep cycle?
A sleep cycle is a repeating unit of about 90 minutes that your brain and body pass through while you sleep. Each cycle begins with light sleep (stages 1-2), descends into deep sleep (slow-wave sleep, stage 3), and ends with dreaming REM sleep. A typical night repeats 4 to 6 of these cycles.
Why wake at the end of a cycle
If an alarm goes off during deep sleep, your brain can't shift into a fully alert state, causing sleep inertia — that groggy, tired feeling even after a full night. Wake up instead when a cycle ends and you've risen into light sleep, and you'll feel far fresher even with slightly less total sleep.
This calculator uses the 90-minute cycle to work backward from your wake-up or bedtime, finding the points where a cycle finishes and suggesting the best times accordingly.
The formula
The formula for working back from your wake-up time to your bedtime is:
Bedtime = Wake-up time − (cycles × 90 min) − 15 min
Here, 90 minutes is the length of one sleep cycle, and 15 minutes is the average time it takes to fall asleep. Working from a bedtime instead, the formula becomes Wake-up time = Bedtime + 15 min + (cycles × 90 min).
Example
If you need to wake at 7:00 AM and want 5 cycles (7 hours 30 minutes): 7:00 − (5 × 90 min = 450 min) − 15 min = bedtime of 11:15 PM. For 6 cycles (9 hours), go to bed at 9:45 PM; for 4 cycles (6 hours), at 12:45 AM.