How Sleep Cycles Work
A single sleep cycle lasts about 90 minutes and moves through several stages: light sleep, deep sleep, and REM (rapid eye movement) sleep. Most adults go through 4 to 6 full cycles per night.
Waking up between cycles, rather than in the middle of one, is what makes you feel alert instead of groggy. This calculator times your sleep so you wake up at the end of a complete cycle.
How Many Hours of Sleep Do You Need?
- Adults (18-64): 7 to 9 hours (5-6 cycles)
- Older adults (65+): 7 to 8 hours (5 cycles)
- Teenagers (14-17): 8 to 10 hours (5-6 cycles)
The calculator adds about 15 minutes to account for the time it takes most people to fall asleep. Actual sleep onset varies from person to person.