A researcher gives a healthy volunteer a drug that completely blocks melatonin synthesis. Which of the following effects on sleep architecture would be MOST expected?
- A Increased sleep onset latency and disruption of circadian phase-shifting without altering total sleep time significantly ✓
- B Complete insomnia due to loss of the primary sleep-promoting signal
- C Loss of REM sleep, since melatonin is the REM-triggering hormone
- D Loss of NREM slow-wave sleep as melatonin drives slow-wave oscillations
Explanation
Melatonin from the pineal gland (secreted in darkness under SCN control) acts primarily as a circadian timing signal, not as a direct sleep-driving substance. Blocking melatonin synthesis impairs circadian phase synchronization, increasing sleep onset latency and making it harder to maintain phase-appropriate sleep, but does not abolish sleep entirely because the homeostatic sleep drive (Process S, mediated by adenosine accumulation) remains intact. Total sleep time is only modestly reduced. Melatonin does not trigger REM or slow-wave sleep specifically (options C and D). Complete insomnia (option B) would occur only if the VLPO and homeostatic systems were simultaneously disrupted.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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