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# Navigating the stages of sleep

## Problem

As you sleep, your brain and body cycle through varying levels of brain wave activity and eye movement; even muscle tone and core body temperature change throughout the night. These levels can be broken down into four major stages: N1, N2, N3, and REM. REM refers to rapid eye movement, a major characteristic of this stage. REM is the stage where your most vivid dreams occur and is required to feel fully rested. It is also characterized by atonia caused by inhibition of the motor neurons.
N1, N2, and N3 are all non-rapid-eye-movement sleep stages, also called synchronized sleep, due to the nature of electroencephalogram (EEG) recordings during these periods. N1 is the onset of sleep, and when you experience “hypnic jerks” – often associated with the feeling of falling. N2, light sleep, is where you spend about 50, percent of your night. N3, known as deep sleep, is characterized by very slow brain waves. Sleepwalking, also known as somnambulism, occurs chiefly during this sleep stage.
As you fall asleep, you pass through N1, N2, N3, and back to N2 before achieving REM sleep. This typically occurs about 90 minutes after falling asleep, and the first REM will last about 20 minutes. As the night progresses, one spends more time in REM sleep, and less in N3. The average night of sleep includes five of these cycles.
Figure 1. A hypnogram tracking the stages of sleep throughout the night.
What is atonia?