In a healthy young adult falling asleep at night, how long after sleep onset does the first episode of REM sleep typically appear?

• A About 90 minutes ✓
• B 30 to 40 minutes
• C 5 to 10 minutes
• D About 3 hours

Explanation

The normal REM latency in adults is approximately 90 minutes, after which REM periods recur roughly every 90 minutes through the night. Markedly reduced REM latency below about 20 minutes suggests narcolepsy or endogenous depression, while prolonged latency occurs with alcohol and benzodiazepine use. Options C and B correspond to early NREM cycling, not the first REM entry.

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

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