Which hormone is directed by the brain's sleep-wake center to promote sleep onset?

• A. Insulin
• B. Melatonin
• C. GABA
• D. Cortisol

Answer: (B)

Melatonin is the hormone directed by the brain's sleep-wake center to promote sleep onset. The brain's circadian system, using light-dark cues, tells the pineal gland to release melatonin when it gets dark. Rising melatonin levels signal the body that night has begun, helping to lower alertness and ease the transition into sleep. Light exposure at night can suppress melatonin and delay sleep. Insulin governs blood glucose, not sleep onset; GABA is a neurotransmitter that promotes sleep but isn’t the sleep-wake center’s hormonal signal; cortisol rises to promote wakefulness in the morning, not the onset of sleep.