Nonrapid eye movement (NREM) is a stage of sleep where the eyes are motionless, and there is no rapid eye movement (REM) activity. This stage of sleep is characterized by low brain activity, muscle relaxation, and a decrease in metabolic rate. There are several synonyms for nonrapid eye movement, including slow-wave sleep, deep sleep, and restorative sleep. Slow-wave sleep refers to the slow, oscillating brainwaves that occur during NREM sleep. Deep sleep emphasizes the restorative aspect of NREM, as this stage of sleep is critical for physical and mental restoration. Restorative sleep highlights the importance of NREM in maintaining overall health and well-being.