What is the main characteristic of Stage 4 sleep as indicated by EEG?

The main characteristic of Stage 4 sleep, also known as deep sleep or slow-wave sleep, is the predominance of delta waves as indicated by EEG. Delta waves are characterized by low frequency (0.5 to 4 Hz) and high amplitude brain waves, which reflect the brain's decreased level of activity during this stage.

In this state, the body experiences the most restorative processes, such as tissue growth and repair, immune system strengthening, and energy replenishment. The presence of delta waves is essential for these restorative functions and promotes overall health.

The other characteristics of sleep stages, like high-frequency brainwaves or theta waves, are more associated with lighter sleep stages or different stages such as REM (rapid eye movement) sleep, where the brain exhibits mixed frequency patterns, including some high-frequency activity. However, during Stage 4, the EEG clearly shows predominantly delta wave activity, making this the defining feature of this deep sleep stage.