How it works
A typical sleep cycle lasts about 90 minutes, moving from light sleep through deep sleep and into REM. Waking near the end of a cycle tends to feel easier than waking from deep sleep in the middle of one.
This calculator uses 90-minute cycles and adds a fall-asleep buffer. If you tell it when you want to wake up, it counts backwards in 90-minute chunks to suggest four bedtimes. If you tell it when you are going to bed, it counts forwards to suggest four wake-up times.
The four results give you options from roughly 6 to 9 hours of sleep, depending on how much time you have.
Limitations
Ninety minutes is an average; real cycles vary from person to person and across the night. Sleep quality, age, alcohol, medications, and sleep disorders change how restorative sleep is. Use this as a planning aid, not a guarantee of waking refreshed.