deep sleep

Meaning: Deep sleep is slow-wave sleep, the part of non-REM sleep with the lowest arousal. Older textbooks split it into stages 3 and 4. The newer scoring system merges them into stage N3. The EEG shows delta waves, which are high in amplitude and low in frequency because large groups of neurons fire in synchrony. Heart rate, breathing and muscle tone fall, and a sleeper is hardest to wake. Growth hormone release is highest in this stage. Deep sleep is concentrated in the first half of the night, while REM periods lengthen toward morning. Older adults spend less time in it.

On the MCAT: A passage gives an EEG trace or a sleep-cycle graph (hypnogram), and you match the wave pattern to the stage. Awake and alert shows beta waves, relaxed with eyes closed shows alpha, light sleep shows theta, and stage 2 adds sleep spindles and K complexes. Deep sleep shows delta waves. Questions may ask which stage sleepwalking, night terrors or bedwetting tend to occur in. They may also ask how the stage proportions change across the night or with age.

Common confusion: Deep sleep is not REM sleep. REM is called paradoxical sleep because its EEG looks like waking, and it brings vivid dreams and loss of muscle tone. Deep sleep has the slowest and most synchronized waves. Nightmares are linked to REM, but night terrors are linked to deep sleep.