Final answer:
During sleep, eye movements are especially significant during REM sleep, characterized by rapid movement under closed eyelids, unlike during NREM sleep when eyes are typically still. These movements are controlled by strong external eye muscles and are thought to be related to dreaming and memory consolidation.
Step-by-step explanation:
Different stages of sleep involve distinct physiological and neurological changes, including eye movements. During REM sleep, which usually occurs after the initial Non-REM (NREM) stages, the eyes exhibit rapid movements under closed eyelids.
These movements are called rapid eye movements and are one of the defining characteristics of REM sleep. The external eye muscles, which are some of the strongest muscles in the body relative to the work they do, are responsible for these eye movements.
However, during NREM sleep, which consists of stages 1 through 3, there is no significant eye movement; the eyes are typically still.
NREM sleep involves a decrease in brain wave frequency and an increase in amplitude as we progress from stage 1 to the deeper sleep stages like stages 3 and 4, which are characterized by delta waves, often referred to as slow-wave sleep.
The role of eye movements during REM sleep is not entirely understood, but it correlates with brain waves similar to those during wakefulness and the occurrence of dreaming. Furthermore, this phase of sleep, as well as NREM sleep, seems to be associated with important aspects of learning and memory consolidation.