Throughout your sleep, your brain cycles repeatedly through two different types of sleep: REM (rapid-eye movement) sleep and non-REM sleep.
The first part of the cycle is non-REM sleep, composed of four stages. The first stage occurs between being awake and falling asleep. The second is light sleep when heart rate and breathing are regulated and body temperature drops. The third and fourth stages are deep sleep. Though REM sleep was previously believed to be the most important sleep phase for learning and memory, newer data suggests that non-REM sleep is more important for these tasks and is also the more restful and restorative phase of sleep.
As you cycle into REM sleep, the eyes move rapidly behind closed lids, and brain waves are similar to those during wakefulness. Breath rate increases and the body becomes temporarily paralyzed as we dream.
The cycle then repeats itself, but with each cycle, you spend less time in the deeper stages three and four of sleep and more time in REM sleep. On a typical night, you’ll cycle through four or five times.