The sleep cycle is approximately 90 minutes in duration. In which part of the night is slow-wave sleep (N3) maximally represented?

• A Middle portion of the night
• B Last third of the night
• C First third of the night ✓
• D Equally distributed throughout the night

Explanation

Slow-wave sleep (N3) predominates in the first third of the night (first 2–3 sleep cycles), reflecting homeostatic sleep pressure accumulated during wakefulness. REM sleep, in contrast, becomes progressively longer and more intense in the later cycles (last third of the night), driven by circadian REM-promoting mechanisms. This distribution has clinical implications: early-night sleep deprivation impairs restorative SWS, while late-night loss (from early morning awakening) reduces REM.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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