Work out the best time to go to bed or wake up, based on full 90-minute sleep cycles — so you wake up feeling rested, not groggy.
1 tool in this categorySleep happens in roughly 90-minute cycles moving through light sleep, deep sleep, and REM. Waking up in the middle of a cycle — especially deep sleep — is what makes you feel groggy even after 8 full hours. Waking up between cycles tends to feel far more refreshing, even with a slightly shorter total sleep time.
Instead of just aiming for "8 hours," this calculator works backward or forward in 90-minute blocks from either your wake-up time or your bedtime, giving you a few optimal windows to fall asleep or set your alarm — so you're more likely to wake up between cycles rather than in the middle of one.
Alongside the Sleep Cycle Calculator, the Sleep Debt Calculator shows how much sleep you have actually missed this week, and the Nap Calculator helps you time a nap to avoid grogginess.
Cycle length varies a little from person to person (roughly 80–120 minutes), and it also depends on how quickly you actually fall asleep after getting into bed. Treat the suggested times as a strong starting point, and adjust based on how you actually feel over a week or two.