Answer:
sleep periods in which a person's eyes move rapidly
Explanation:
In REM (Rapid eye movement stage of sleep) a sleeping person has a conscious mind due to which he/she is able to sense his/her dream to be taking place in reality. The brain of a person in this sleep stage is as active as the brain of an awake person.
Due to this conscious dreaming, a person eyes move very fast which is also known as darting of eyes.
Thus, REM sleep refers to sleep periods in which a person's eyes move rapidly
