Sleep happens in cycles of roughly 90 minutes, moving through light sleep, deep sleep, and REM. Waking up in the middle of a deep stage is what leaves you groggy; waking at the end of a cycle feels far more refreshing.
This calculator counts back from your wake-up time in 90-minute blocks and adds about 15 minutes to fall asleep, suggesting bedtimes that let you complete 4, 5, or 6 full cycles (roughly 6–9 hours of sleep). Most adults feel best with 5–6 cycles per night.
Most adults need 7–9 hours, which is about 5 to 6 full 90-minute sleep cycles. Teenagers and children need more. Aim to wake at the end of a cycle rather than the middle.
Waking at the end of a 90-minute cycle, when sleep is lightest, helps you feel alert. Waking mid-cycle from deep sleep tends to cause grogginess, even after the same total hours.
Most people take around 10–20 minutes to actually fall asleep after getting into bed. The calculator adds 15 minutes so the suggested bedtime reflects when you lie down, not when you're already asleep.
Try the next bedtime option to add or remove a full cycle, keep a consistent schedule, and limit screens and caffeine before bed. Persistent fatigue despite enough sleep is worth discussing with a doctor.