Sleep Cycles (~90 Minutes)
Sleep progresses through NREM stages (light → deep) and REM (dreaming). A full cycle is often ~90 minutes. Waking between cycles can feel easier than deep sleep mid-cycle — hence "smart alarms."
Individual cycle length varies — 90 min is an average.
Sleep Duration Guidelines
National Sleep Foundation broadly suggests 7–9 hours for 18–64 adults — not a rigid prescription. Teens need more; older adults may sleep less at night but nap.
Chronic short sleep links to metabolic, immune, and cognitive risk — prioritize consistency and dark, cool rooms.
Bedtime Math
If you need to wake at 6:30 AM and want 5 full cycles (5 × 1.5h = 7.5h), count back 7h30m — 11:00 PM bedtime.
Add ~15 minutes to fall asleep if you're not instant — adjust experimentally.
How to Use This Sleep Calculator
Enter wake time or bedtime and desired sleep duration or cycles. See suggested paired times to experiment with — track energy for a week to validate.