How sleep cycles work
Through the night you move through cycles of light, deep, and REM sleep, each about 90 minutes. Waking mid-cycle feels groggy; waking at the end feels alert. This counts back in 90-minute blocks (plus ~15 minutes to fall asleep) from your wake time. Worked example: for a 6:30 am wake time, five full cycles (7.5 hours) puts bedtime at roughly 10:45 pm, and six cycles (9 hours) at roughly 9:15 pm.
Where the 90-minute figure comes from
Sleep researchers Nathaniel Kleitman and Eugene Aserinsky first mapped these recurring cycles using EEG recordings in the 1950s, identifying the alternation between non-REM and REM stages that repeats through the night. Later research refined the average cycle length to roughly 90 minutes, though it is an average, not a fixed number.
How much sleep do you need?
- Adults: 7–9 hours (5–6 cycles)
- Teens: 8–10 hours
- Older adults: 7–8 hours
Sleep deeper
- Keep a consistent sleep/wake time.
- Dim lights and screens 30–60 min before bed.
- Keep the room cool, dark, quiet.
- Avoid late caffeine and big meals.
Limitations
Individual cycle length actually ranges from about 70 to 120 minutes and varies across the night, so this calculator gives a reasonable estimate to aim for, not an exact prediction of when you'll naturally wake up.