The term 'sleep cycle' refers to the natural pattern of sleep and wakefulness that humans experience throughout the day. It is composed of two main stages: rapid eye movement (REM) and non-rapid eye movement (NREM). REM sleep is a deep sleep stage characterized by rapid eye movements, increased brain activity, and vivid dreaming. NREM sleep is a lighter sleep stage characterized by slower brain activity and less dreaming. Together, these two stages form the sleep cycle, which is essential for physical and mental health.