Find the best bedtime or wake time based on 90-min sleep cycles.
Sleep occurs in 90-minute cycles, each containing light sleep, deep sleep, and REM sleep. Waking mid-cycle leaves you feeling groggy. Timing your alarm to the end of a complete cycle lets you wake naturally at the lightest stage of sleep.
Most adults need 5–6 complete cycles (7.5–9 hours) per night. REM sleep, which supports memory consolidation and emotional processing, is most abundant in the final cycles before waking — another reason cutting sleep short is costly.