Non-rapid eye movement (NREM) sleep

Non-rapid eye movement (NREM) sleep is one of the main stages of sleep, characterized by the absence of rapid eye movements. It is further divided into three sub-stages: N1, N2, and N3. NREM sleep typically occurs during the first half of the sleep cycle and is associated with different physiological and brain activity compared to rapid eye movement (REM) sleep.

Here are the characteristics of each NREM sleep stage:

N1 (Stage 1): N1 is the transition stage between wakefulness and sleep. During this stage, the brain produces theta waves, which are slower in frequency compared to the alpha waves of wakefulness. N1 sleep is relatively light, and people can be easily awakened during this stage. It usually lasts for a few minutes.
N2 (Stage 2): N2 is a deeper stage of sleep compared to N1. It is characterized by the presence of sleep spindles and K-complexes on an electroencephalogram (EEG), which represent brief bursts of brain activity. The body temperature decreases, heart rate slows down, and eye movements cease during N2 sleep. This stage constitutes a significant portion of total sleep time and plays a role in memory consolidation.
N3 (Stage 3): N3, also known as slow-wave sleep (SWS) or deep sleep, is the deepest and most restorative stage of NREM sleep. It is characterized by slow delta waves on an EEG, which have the slowest frequency. During N3 sleep, blood pressure drops, breathing slows down, and the body undergoes physiological repairs and growth. It is particularly important for physical restoration, immune function, and overall well-being. N3 sleep is also critical for memory consolidation, particularly for consolidating declarative (fact-based) memories.

Throughout the night, the sleep cycle repeats several times, with each cycle comprising NREM sleep followed by REM sleep. The proportion of time spent in each NREM stage can vary, with a higher amount of N3 sleep occurring earlier in the night and more REM sleep occurring later.

Disruptions to NREM sleep, such as sleep disorders or sleep deprivation, can have negative effects on physical and cognitive health. Adequate and uninterrupted NREM sleep is essential for overall sleep quality and plays a crucial role in rest, recovery, memory consolidation, and overall well-being.