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Find out what’s happening in your brain while you’re fast asleep.

Your body may be still while you sleep, but your brain is actually pretty active. In fact, it goes back and forth between two phases: rapid eye movement (REM) sleep, which is when dreams typically happen, and non-rapid eye movement (NREM) sleep, which is when the body bolsters your immune system, builds muscle and bone, and mends and regenerates tissues. Your brain first enters four different stages of NREM sleep and then goes through REM, and this whole pattern is considered one cycle. The cycles repeat themselves until you wake up—usually after about five full cycles. Find out what goes on step-by-step, below.

Stage OneThis is the lightest sleep stage when you can be easily awoken and where you may feel like you’re falling or have sudden muscle twitches (also known as “hypnic jerks”).
Stage TwoDuring this second stage, your heart rate slows, your muscles contract and relax, and your body temperature decreases as you prepare to go into deep sleep.
Stages Three and FourYou’ve now entered the deep sleep phases where your brain waves slow down, and it becomes more difficult to wake up.
REMIn this stage, your heart rate, breathing, and eye movement all speed up and your brain becomes more active, processing things that you’ve learned during the day to help you form memories and boost feel-good chemicals, such as serotonin.