Slow-wave sleep

Meaning: Slow-wave sleep is stage N3 of non-REM sleep. Large groups of cortical neurons fire in synchrony, producing delta waves on an EEG: high amplitude and low frequency (roughly 0.5 to 4 Hz). Heart rate, breathing and muscle tone are low, and sleepers are hardest to rouse. Older texts split it into stages 3 and 4. It is linked to physical restoration and memory consolidation, and growth hormone is released in pulses during it. The amount declines with age.

On the MCAT: Passages show EEG tracings or a sleep-cycle graph (hypnogram). You identify the stage from the waves: alpha waves for relaxed wakefulness with eyes closed, theta waves with spindles and K complexes in stage 2, delta waves in N3, and fast, irregular waves in REM. A cycle lasts about 90 minutes. Across the night, N3 shortens and REM lengthens, so slow-wave sleep dominates the first half. Sleepwalking, night terrors and bedwetting are classic parasomnias tied to slow-wave sleep.

Common confusion: Students assume the deepest sleep is when we dream. Vivid dreaming and muscle atonia are typical of REM sleep, whose EEG looks like waking (hence "paradoxical sleep"). Night terrors arise from slow-wave sleep, while nightmares usually arise from REM. "Slow" describes the EEG frequency, not a quiet brain.