Sleep Cycle Calculator: Why 7.5 Hours Beats 8

Sleep cycles through stages — light sleep → deep sleep → REM → light again — every 85-110 minutes (~90 average). Most adults need 4-6 cycles per night = 6-9 hours. Critically, waking at the END of a cycle (in light sleep) feels refreshed, while waking in the MIDDLE of deep sleep feels groggy regardless of total hours. Practical use: if you must wake at 7am, count back in 90-min cycles: bedtimes of 11:30pm (5 cycles), 10pm (6 cycles), or 8:30pm (7 cycles) all leave you in light sleep at wake-up. Falling-asleep latency is ~14 minutes on average, so go to bed 15 minutes before. Use our Sleep Calculator for the math.