Sleep is a complex enigma that researchers still have yet to crack. However, they do know that a lot occurs inside the body when sleeping – namely, that it cycles through REM and non-REM (NREM) phases of sleep.

There are three stages to NREM sleep, which comes before REM sleep. Each stage during NREM typically lasts between 5 to 15 minutes, and the body will go through each of them before transitioning into REM. During the first stage, the eyes are closed, and the body has just fallen into a void, becoming tired; it is easy to wake a person in this stage. Upon falling into a light sleep, the body enters phase two. 

As the heart rate slows, the body temperature drops as it prepares to go into complete relaxation mode. Deep sleep occurs in the third stage of NREM. It’s difficult to rouse a person who is in this stage and, when it does happen, the person remains hazy and disoriented for a few minutes. NREM sleep is crucial because during these stages the body repairs itself; regrowing tissues, building muscles and bones, and strengthening the immune system. 

REM, or “rapid eye movement,” sleep is a phase categorized by the eyes moving quickly in different directions. This does not happen in the three NREM stages. After the three NREM stages are complete, the body enters this dreaming (REM) stage. Breathing becomes shallow, the eyes dart around behind the eyelids, body temperature is less easily maintained, and muscles become temporarily paralyzed to keep the person sleeping from acting out his/her dreams. Brain waves are very active during this stage, and awakenings, as well as arousal, occur easily. During this phase, people can spring to life from a nightmare. REM sleep is shorter than NREM sleep, taking up only a small percentage of total minutes slept.

The body cycles through NREM and REM several times per night, with one sleep cycle consisting of NREM through REM. The process is repeated until the person wakes up. While the first full cycle typically lasts for about 90 minutes, the subsequent cycles are closer to 100 or 120 minutes. On average, an individual cycles through four or five times each night.