Each sleep cycle lasts about 90 minutes and progresses through four stages. Waking between cycles — not in the middle of deep sleep — is the key to feeling refreshed.
Stage Breakdown
Stage 1: Light Sleep (1-5 min)
Transition from wakefulness to sleep. Easy to wake. Body begins to relax, heart rate slows.
Stage 2: Deeper Light Sleep (10-25 min)
Body temperature drops, heart rate slows further. Brain produces sleep spindles. Takes up about 45% of total sleep.
Stage 3: Deep Sleep (20-40 min)
Slow-wave sleep. Body repairs itself, immune system strengthens, growth hormone released. Very hard to wake. Waking here causes grogginess.
Stage 4: REM Sleep (10-60 min)
Dreaming stage. Brain is nearly as active as when awake. Memory consolidation and emotional processing. First REM period is short; each cycle's REM gets longer.
The 90-Minute Rule
If you need to wake at 7:00 AM, count backward in 90-minute increments:
5 cycles
Sleep by 11:30 PM
7.5 hours — Ideal
6 cycles
Sleep by 10:00 PM
9 hours — Great
4 cycles
Sleep by 1:00 AM
6 hours — Minimum
Waking at the end of a cycle (rather than in the middle of deep sleep) makes a dramatic difference in how you feel. Add about 15 minutes to fall asleep when calculating your bedtime.
Find Your Optimal Bedtime
Use our Sleep Calculator to calculate the best bedtime based on when you need to wake up, factoring in sleep cycles and the time it takes you to fall asleep.