Which hormone exhibits diurnal variation and plays a key role in regulating the sleep-wake cycle?

• A. Melatonin
• B. Cortisol
• C. Prolactin
• D. Growth hormone

Answer: (B)

Melatonin is the hormone that shows diurnal variation and is central to regulating the sleep-wake cycle. It’s produced by the pineal gland, and its secretion is controlled by the light-dark cycle via signals from the brain’s circadian clock. When it gets dark, melatonin levels rise, promoting sleepiness; they peak at night and fall toward morning, helping synchronize sleep with night and day. Cortisol also varies daily and helps with wakefulness and energy in the morning, but it’s not the primary regulator of sleep itself. Prolactin and growth hormone have their own patterns related to sleep, but they don’t drive the core sleep-wake rhythm like melatonin does.