What hormone is produced in response to darkness to regulate circadian rhythms?

• A. Ghrelin
• B. Insulin
• C. Adrenaline
• D. Melatonin

Answer: (D)

Melatonin is produced by the pineal gland in response to darkness to regulate circadian rhythms. When light fades, signals from the retina to the brain’s master clock peak at night and stimulate the pineal gland to release melatonin. This rise in melatonin promotes sleepiness and helps synchronize the body's internal clock with the day-night cycle. Light during the day suppresses melatonin, supporting wakefulness. The other hormones have different roles: ghrelin signals hunger, insulin controls blood glucose, and adrenaline is tied to immediate arousal and stress, not the daily timing of sleep.