Sleep and Aging

Frequently Asked Questions

7. How many types of sleep are there?

There are two types of sleep: non-rapid eye movement -- or NREM sleep -- and rapid eye movement -- or REM sleep. NREM sleep has four stages ranging from light to deep sleep. We cycle through these four stages of sleep approximately every 90 minutes. Then we go into REM sleep, the most active stage of sleep when dreaming often occurs. During REM sleep, the eyes move back and forth beneath the eyelids and muscles become immobile.