Which gland secretes melatonin and regulates sleep-wake cycles?

• A. Hypothalamus
• B. Pancreas
• C. Thyroid gland
• D. Pineal gland

Answer: (D)

Melatonin is produced by the pineal gland, a small endocrine gland in the brain. It converts serotonin into melatonin, with its levels rising in the dark and falling in the light. This pattern is driven by the circadian clock in the brain and signals from the retina: when it gets dark, the clock stimulates the pineal gland via the sympathetic nervous system to release melatonin, helping you feel sleepy; light suppresses melatonin to promote wakefulness. The other glands listed don’t produce melatonin—the hypothalamus helps regulate many hormones and contains the clock, but does not secrete melatonin itself; the pancreas makes insulin and glucagon; the thyroid makes thyroid hormones.