Every night when you get into bed, you go through 5 stages of sleep. The first few stages are a light sleep, slowly deepening into stage 3 and 4. After about 90 minutes you enter the 5th stage, REM (Rapid Eye Movement) sleep.

During REM your heart rate and breathing quickens, blood pressure rises and you’re unable to regulate your body temperature as efficiently. Your brain activity also increases to waking level (sometimes even higher).

The rest of your body however, is paralyzed. This is caused by the release of glycine (an amino acid from the brain stem) onto the motoneurons that are responsible for conducting impulses outward from the brain or spinal cord.

Because you dream mostly during this stage of sleep, scientists believe this paralysis is nature’s way of keeping you from acting out your dreams. You wouldn’t really want to fly out of that bedroom window, would you?!