What is the term for the first four stages of sleep, which include the initial phases of non-dreaming sleep?

The term for the first four stages of sleep is indeed nonrapid eye movement sleep, commonly referred to as NREM sleep. This encompasses the initial phases of sleep that occur before reaching REM sleep. NREM sleep is characterized by a lack of rapid eye movements and includes light sleep stages (stage 1 and stage 2) and deeper sleep stages (stage 3 and stage 4).

During these stages, the body gradually transitions from wakefulness into deeper states of relaxation. This is essential for restorative processes such as physical recovery and memory consolidation. The distinction between NREM and REM sleep is significant as REM sleep is where most dreaming occurs, characterized by heightened brain activity whereas NREM sleep focuses on bodily restoration and stabilization of sleep cycles.

The other options represent different aspects of the sleep cycle: REM sleep refers specifically to the rapid eye movement phase and is when dreaming happens, deep sleep generally refers to the further stages of NREM, and stage 5 sleep does not accurately represent a recognized category in the conventional sleep stage model, as classification continues to evolve in sleep research. Therefore, nonrapid eye movement sleep effectively encompasses the full range of the initial four stages.