A: Jenny, there are four stages of sleep, which divide into three NMEN, or non-rapid eye movement stages, and one REM, or rapid eye movement stage.
Stage 1 of NREM sleep: This stage functions as a transition from wakefulness to sleep. During this stage, the body still maintains some tension, so there are brief twitches that take place while the muscles relax. In addition, brain activity, heart rate, and eye movements slow down.
Stage 2 of NREM sleep: This stage works as a subtle sleep period for the body to continue through the following stages. During this phase, there's less awareness of the environment, body temperature reduces, and eye movements stop. Similarly, breathing and muscle tension keep slowing down.
Stage 3 of NREM sleep: This stage results in a deep phase of sleep where most of the necessary rest takes place. During this stage, brain waves known as delta appear, muscles are now completely relaxed, heart rate and breathing continue to slow down.
Stage 4 of REM sleep: This final stage acts as a peak stage for brain activity. During this stage, atonia (or the immobilization of the muscles) takes place, brain activity is comparable to that of wakefulness, and eye movements appear. Additionally, heart rate, blood pressure, and breathing increase. Also, the event of dreaming occurs.