Sleep calculator — wake up refreshed between cycles
This sleep calculator recommends bedtimes or wake-up times aligned to 90-minute sleep cycles so you are less likely to wake during deep sleep. Enter when you must get up—or when you plan to go to bed—and see several options spanning roughly six, seven-and-a-half, or nine hours of rest. Everything runs locally in your browser. For daytime energy planning, combine results with our calorie calculator and Pomodoro timer for work blocks.
Why sleep cycles matter
A full sleep cycle averages about 90 minutes, moving from light sleep into deeper stages and REM. Waking mid-cycle—especially from slow-wave sleep—often feels like grogginess or “sleep inertia.” Waking at the end of a cycle, during lighter sleep, is usually easier.
Adults commonly need 4–6 cycles per night (about six to nine hours of actual sleep). Needs vary by age, health, and activity; this tool gives scheduling math, not medical advice. If you snore heavily, gasp at night, or never feel rested, talk to a clinician.
How the math works
We assume about 14 minutes to fall asleep after getting into bed. For a target wake time, each suggested bedtime equals:
bedtime = wake time − (cycles × 90 min) − 14 min
Example: to wake at 7:00 AM, five complete cycles (7.5 h sleep) point to roughly 11:16 PM bedtime; four cycles suggest about 12:46 AM. Flip the mode to “going to bed at…” and the tool adds cycles forward to list wake times.
7.5 hours vs 8 hours
Eight hours is a round cultural target, but 7.5 hours equals five 90-minute cycles—often a sweet spot for waking on a cycle boundary. Sleeping 8 hours without aligning to cycles can mean waking during deeper sleep. Prioritize consistent bed and wake times over chasing perfect round numbers.
Sleep hygiene tips
- Keep the bedroom cool, dark, and quiet; limit screens before bed.
- Avoid heavy meals and caffeine late in the day if they disrupt your sleep.
- Get morning light exposure to reinforce circadian rhythm.
- Use the calculator to plan backward from a fixed alarm—not to cut sleep short.
See the FAQ section for cycle counts and 6 AM bedtime examples.