The Sleep Calculator helps you wake up feeling refreshed by timing your sleep around 90-minute cycles. Tell it either when you need to wake up or when you plan to go to bed, and it suggests the best times so that you wake at the end of a sleep cycle rather than in the middle of one.
Waking mid-cycle is what often leaves you groggy, even after plenty of sleep. This calculator adds about 15 minutes to fall asleep and works in complete cycles. It runs entirely in your browser — nothing is stored.
How to use the sleep calculator
- Choose whether you want to set a wake-up time or a bedtime.
- Enter that time.
- Read the suggested times — each represents a whole number of 90-minute sleep cycles.
- Pick the option that gives you enough sleep; the top suggestion allows the most rest.
How sleep cycles work
A full night’s sleep is made up of repeated cycles of about 90 minutes, each moving through light sleep, deep sleep, and REM sleep. Waking at the end of a cycle, when sleep is lightest, tends to feel much easier than being woken from deep sleep.
Most adults need five to six cycles a night — roughly 7.5 to 9 hours. This calculator counts back or forward in whole 90-minute cycles and allows around 15 minutes to drift off, so the suggested times line up with the end of a cycle.
Better sleep habits
- Keep a consistent sleep and wake time, even on weekends.
- Avoid screens, caffeine, and heavy meals close to bedtime.
- Keep your bedroom cool, dark, and quiet.
- Cycle length varies by person, so treat these times as a helpful starting point.
Frequently Asked Questions
What time should I go to bed to wake up at a certain time?
Choose “Wake up at…”, enter your target time, and the calculator suggests bedtimes that end on a complete 90-minute sleep cycle, allowing about 15 minutes to fall asleep.
How many sleep cycles do I need?
Most adults do best with five to six cycles a night, about 7.5 to 9 hours. The calculator shows options for six, five, and four cycles so you can choose what fits.
Why do I wake up tired even after 8 hours?
You may be waking in the middle of a deep-sleep stage. Timing your wake-up to the end of a cycle, as this tool suggests, can help you feel more rested.
Is 90 minutes exact for everyone?
No. Cycle length varies between roughly 80 and 100 minutes and changes through the night, so these times are a guide rather than a precise rule.