Calculate the best time to go to bed or wake up, based on 90-minute sleep cycles — so you wake up between cycles instead of in the middle of deep sleep.
The body doesn't sleep evenly through the night, but goes through repeating sleep cycles of about 90 minutes, each consisting of light sleep, deep sleep and REM sleep (dream sleep). Waking up in the middle of a cycle — especially in deep sleep — often leaves you groggy and tired, even if you've slept a long time. Waking up at the transition between two cycles, on the other hand, feels much more refreshing.
The calculator shows three options — 4, 5 and 6 cycles, corresponding to 6, 7.5 and 9 hours of sleep respectively — so you can choose what fits best into your day.
A full sleep cycle, moving through light sleep, deep sleep and REM sleep, lasts on average around 90 minutes in adults. Most people experience 4-6 such cycles per night, though the length varies somewhat from person to person.
Most people take somewhere between 10 and 20 minutes to fall asleep after going to bed. The calculator uses 15 minutes as an average estimate, so the bedtime becomes more realistic.
Not necessarily. What often matters more for how rested you feel is whether you wake up between two sleep cycles or in the middle of one. 6 hours (4 full cycles) can therefore feel more restful than 6.5 hours that interrupts a cycle.