What are the four stages of non-REM sleep?

The four stages of non-REM sleep are identified as N1, N2, N3, and N4, where each stage represents a different depth and characteristic of sleep.

N1 is the lightest stage of sleep, serving as a transition from wakefulness. It features slow eye movements and a decrease in muscle activity. As the individual progresses into N2, the body experiences a further drop in temperature and heart rate, along with sleep spindles and K-complexes that are crucial for memory consolidation.

N3, often referred to as slow-wave sleep or deep sleep, is important for restorative processes, including tissue growth and repair, immune function, and energy restoration. N4 initially appeared in older sleep classifications, but in modern terminology, it is combined with N3. Therefore, N4 is now generally considered synonymous with N3 in terms of its physiological characteristics.

In this context, stating the four stages of non-REM sleep accurately as N1, N2, N3, and N4 captures the necessary stages of non-REM sleep. This classification helps in understanding sleep architecture and its significance to overall health and well-being.