How long is one sleep cycle?

One sleep cycle is approximately 90 minutes, progressing through NREM stages 1, 2, 3, and REM sleep. Most adults need 5–6 complete cycles (7.5–9 hours) per night.

Why do I wake up tired even after 8 hours of sleep?

Waking mid-cycle (during deep NREM or REM sleep) causes sleep inertia — the groggy feeling on waking. Waking at the end of a 90-minute cycle, when sleep is lightest, leaves you feeling more alert.

Sleep Cycle Calculator – Best Bedtime & Wake-Up Times

Sleep Cycle Calculator

Find the best bedtime or wake-up time to complete full 90-minute sleep cycles and wake up refreshed.

I want to calculate

When to wake up (from bedtime)

When to go to bed (from wake time)

Bedtime (when you go to sleep)

Wake up at one of these times (sleeping at 6:41 PM)

Includes ~14 min to fall asleep

Wake time	Cycles	Sleep
8:25 PM	1 cycles	1.5 hrs
9:55 PM	2 cycles	3.0 hrs
11:25 PM	3 cycles	4.5 hrs
12:55 AM	4 cycles	6.0 hrs
2:25 AM	5 cycles	7.5 hrs	Recommended
3:55 AM	6 cycles	9.0 hrs	Recommended

Waking at the end of a sleep cycle (rather than mid-cycle) means you wake from lighter NREM sleep, feeling more alert. Most adults need 5–6 cycles (7.5–9 hours).