The Science of Sleep Cycles

Sleep is not a uniform state. Throughout the night your brain cycles through light sleep, deep sleep, and REM. Waking during deep sleep (Stage 3) causes grogginess and disorientation, while waking at the end of a cycle — during light sleep or REM — leaves you feeling alert and refreshed.

How Long Should You Sleep?

Most adults need 5 or 6 full sleep cycles per night — 7.5 to 9 hours of actual sleep. Individual needs vary with genetics, daily exertion, and age. This calculator treats 5 and 6 cycles as optimal and includes your sleep latency (time to fall asleep) so the target time is accurate.

Improving Sleep Hygiene

Timing matters, but so does quality. Keep the bedroom cool (around 65°F), avoid blue light from screens an hour before bed, and keep a consistent bedtime every night, including weekends. Waking up at the same time each day anchors your circadian rhythm.