The brain cycles through five distinct phases during sleep: stages 1, 2, 3, 4, and rapid eye movement (REM) sleep. REM sleep makes up about 25% of your sleep cycle and first occurs about 70 to 90 minutes after you fall asleep. Because your sleep cycle repeats, you enter REM sleep several times during the night.
During REM sleep, your brain and body are energized and dreaming occurs.1 REM is thought to be involved in the process of storing memories, learning, and balancing your mood,2 although the exact mechanisms are not well understood.
REM sleep begins in response to signals sent to and from different regions of the brain. Signals are sent to the brain's cerebral cortex, which is responsible for learning, thinking, and organizing information. Signals are also sent to the spinal cord to shut off movement, creating a temporary inability to move the muscles ("paralysis") in the arms and legs. Abnormal disruption of this temporary paralysis can cause people to move while they are dreaming. For example, this type of movement while dreaming can lead to injuries that could happen when a person runs into furniture while dreaming of catching a ball.3
REM sleep stimulates regions of the brain that are used for learning. Studies have shown that when people are deprived of REM sleep, they are not able to remember what they were taught before going to sleep.3 Lack of REM sleep has also been linked to certain health conditions, including migraines.2
The reason for dreaming during REM sleep is not understood. While some of the signals sent to the cortex during sleep are important for learning and memory, some signals seem to be random. It is these random signals that may form the basis for a "story" that the brain's cortex tries to interpret or find meaning in, resulting in dreaming.3
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