Based on 90-minute sleep cycle theory. Calculate the best bedtime or wake-up time so you wake up during light sleep — refreshed and energized.
Each cycle lasts ~90 minutes: light sleep → deep sleep → REM. Waking at cycle end (light sleep) = refreshed. Waking during deep sleep = groggy (sleep inertia).
School-age: 9-11h | Teens: 8-10h | Adults: 7-9h | Seniors: 7-8h. 5-6 cycles (7.5-9h) is ideal for most adults.
Consistent schedule, no screens before bed, cool dark bedroom, no evening caffeine/alcohol, daytime sunlight exposure.