Sleep is triggered by a group of hormones in the body. These hormones react to cues within the body and in the environment. There is an approximate 80 percent of sleep time without dreams. This stage of dreamless sleep is referred to as NREM which stands for non-rapid eye movement. Dreams occur primarily during rapid eye movement or REM.

During non-rapid eye movement sleep or NREM, the breathing and heart rates are slow and the blood pressure goes down. Based on the Rechtschaffen and Kales standardization of the year 1968, NREM was divided into 4 stages. However, it has been reduced to 3 by The American Academy of Sleep Machine in 2007.