Sleep Calculator
Find the best bedtime or wake-up time based on natural 90-minute sleep cycles.
How sleep cycles work
Sleep happens in cycles of roughly 90 minutes, moving through light sleep, deep sleep and REM sleep. Waking up in the middle of a cycle - especially during deep sleep - tends to leave you feeling groggy, while waking up at the end of a complete cycle usually feels more refreshing. This calculator adds an average of 14 minutes to fall asleep, then counts backward or forward in 90-minute blocks.
How many cycles do I need?
- 5 cycles (7.5 hours) and 6 cycles (9 hours) are generally recommended for most adults
- 4 cycles (6 hours) can work occasionally but isn't a substitute for regular full-length sleep
- Individual sleep needs vary - use this as a starting point, not an exact rule
Frequently asked questions
Why 90-minute cycles specifically?
Sleep researchers have found that a complete sleep cycle - moving through all sleep stages and back to light sleep - averages about 90 minutes in adults, though it can range from 70 to 120 minutes for a given person and night.
Where does the 14-minute fall-asleep time come from?
It's a commonly used average for how long a typical adult takes to fall asleep after getting into bed. Your personal time may be shorter or longer, so treat the suggested times as a helpful estimate rather than an exact schedule.
Is waking up mid-cycle actually harmful?
It's not harmful to your health, but it commonly causes sleep inertia - that groggy, disoriented feeling right after waking - which tends to be milder when you wake up between cycles instead of during deep sleep.