Sleep Cycle calculator.
Wake up at the end of a 90-minute cycle, not the middle of one. The difference is the difference between groggy and clear.
Go to bed at one of these times
Calculated as 90-minute sleep cycles plus a 14-minute average time to fall asleep. Waking at the end of a cycle (rather than the middle) is why you sometimes feel worse after a longer nap.
How it works.
Sleep happens in repeating ~90-minute cycles of light, deep, and REM stages. If your alarm fires while you're mid-cycle — especially mid-deep-sleep — you'll feel groggy for 30+ minutes. This phenomenon is called sleep inertia.
This calculator works backward from your target wake time, in 90-minute increments, plus a 14-minute average for falling asleep. The result is a list of bedtimes where your alarm will fire at a cycle boundary — the moment your brain is naturally primed to wake.