A full night of sleep runs through several roughly 90-minute cycles of light, deep, and REM sleep. Waking up at the end of a cycle, rather than in the middle of one, tends to feel noticeably less groggy, even with the same total time asleep.
Use this when you have a fixed wake-up time and want to work backward to a bedtime that ends a sleep cycle rather than interrupting one, or the reverse when you know what time you're getting into bed tonight. It's also handy for shift workers or after travel, when your usual sleep timing has been thrown off and you're rebuilding a schedule from scratch.
Each sleep cycle runs about 90 minutes. This calculator adds a 15-minute buffer for the time it typically takes to fall asleep, then counts back or forward in 90-minute blocks (6, 5, 4, and 3 cycles) to suggest bedtimes or wake times that land at the end of a cycle rather than in the middle of one.
Health Calc