Practice Passage Test - 12 | Practice Passages for MCAT PDF Download

During the sleep cycle, various changes occur in the brain and body, including fluctuations in brain wave activity, eye movement, muscle tone, and core body temperature. These changes can be categorized into four main stages: N1, N2, N3, and REM. The REM stage is characterized by rapid eye movement and is when vivid dreams occur, contributing to a feeling of being fully rested. Inhibition of motor neurons causes atonia during REM sleep. On the other hand, N1, N2, and N3 are non-rapid-eye-movement sleep stages, also known as synchronized sleep, as observed in electroencephalogram (EEG) recordings. N1 marks the onset of sleep and is associated with "hypnic jerks" and a falling sensation. N2 is a stage of light sleep, accounting for approximately 50% of the night. N3, referred to as deep sleep, is characterized by slow brain waves and is the stage during which sleepwalking, or somnambulism, mainly occurs. The sleep cycle progresses from N1 to N2, N3, and back to N2 before entering REM sleep, which typically happens around 90 minutes after falling asleep. The first REM period lasts about 20 minutes, and as the night goes on, more time is spent in REM sleep and less in N3. On average, a night of sleep consists of five cycles. Please refer to Figure 1 for a hypnogram illustrating the stages of sleep throughout the night.