Sleep occurs in cycles of two phases: rapid eye movement (REM) and non-rapid eye movement (NREM) sleep, which is further divided into three stages, N1-N3. Each phase and stage of sleep includes variations in muscle tone, brain wave patterns, and eye movements. The body cycles through all of these stages approximately 4 to 6 times each night, averaging 90 minutes for each cycle. The stages of sleep are as follows:

• Wake: This is the stage when you are fully awake and alert.

• N1: This is the lightest stage of sleep and begins when more than 50% of the alpha waves are replaced with low-amplitude mixed-frequency (LAMF) activity. Muscle tone is present in the skeletal muscle, and breathing tends to occur at a regular rate.

• N2: This is still light sleep, but deeper than stage N1. During this stage, your brain waves slow down and have noticeable pauses between short, powerful bursts of electrical activity. Experts think those bursts are your brain organizing memories and information from the time you spent awake.

• N3: This is the deep sleep stage, also known as slow-wave sleep (SWS) or delta sleep. It is harder to wake someone up if they are in this phase. Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relaxes even further. This is the stage when the body repairs and regrows tissues, builds bone and muscle, and strengthens the immune system.

• REM: This is the stage when you dream. During REM sleep, your eyes move rapidly, and your brain activity increases to levels similar to when you are awake. Your heart rate and breathing quicken, and your muscles become temporarily paralyzed. REM sleep is important for learning, memory consolidation, and emotional regulation.

It is important to note that the length of each stage and the number of cycles can vary from person to person and from night to night based on a wide range of factors such as age, recent sleep patterns, and alcohol consumption.