Our bodies are usually inactive during sleep, so while we may have little awareness of the external world, our brains continue to work. The brain goes deeper into stages of sleep towards low activity. After around 90 minutes, however, our brain activity switches direction and becomes similar to what it would be if we were awake. We can’t move because our muscles are relaxed, and we generally feel tired and drowsy; this stage is known as REM sleep. When we dream at night, it is usually during this state. It’s characterized by Rapid-Eye Movements and brain activity akin to being awake.
We typically go through two or three cycles of alternating REM and non-REM (non-rapid-eye-movement) sleep each night, and every time the non-REM periods get shorter and shallower, while the periods of REM sleep get longer and deeper. Lack of REM sleep has also been shown to cause ‘REM rebound,’ where people stay awake longer and have more REM dreams later in the cycle. This made psychologists assume that perhaps the purpose of sleep is not only to rest but also to dream and prepare our brains for fresh information after sorting stimuli from being awake.
