What term describes quiet, typically dreamless sleep without rapid eye movements, divided into four stages?

NREM sleep, or Non-Rapid Eye Movement sleep, is the correct term for the type of sleep described in the question. This phase of sleep is characterized by the absence of rapid eye movements and is typically associated with deep, restorative sleep. It is divided into four distinct stages, ranging from light sleep (Stage 1) to deep sleep (Stage 4), with each stage playing a crucial role in overall sleep quality and health.

During NREM sleep, especially in the deeper stages, various physiological changes occur, such as a decrease in heart rate, blood pressure, and breathing rate. This contributes to physical restoration and immune functioning. The dreamless quality of NREM sleep, particularly in the deeper stages, helps differentiate it from REM sleep, where most vivid dreaming occurs and eye movements are present.

The other terms listed refer to different aspects of sleep. Deep sleep often refers to the lower stages of NREM sleep but does not encompass all four stages as a whole. Hypnagogic sleep is a transitional state occurring between wakefulness and sleep and is not specifically a sleep stage. REM sleep, distinct from NREM sleep, is marked by rapid eye movements and heightened brain activity, often associated with vivid dreams.