Why sleep cycles matter
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 deep sleep) tends to feel groggy, while waking up between cycles usually feels more refreshing — even with the same total sleep time.
How this calculator works
It counts backward or forward in 90-minute blocks from your chosen time, adding about 15 minutes for the average time it takes to fall asleep, and suggests a few options so you can pick what fits your schedule.
Individual sleep needs vary. This is a general guide based on average cycle length, not a substitute for personal experience or medical advice.