The hormone melatonin, which regulates the sleep-wake cycle, is secreted by the:

The pineal gland is a small, pine-cone-shaped endocrine gland located deep in the brain, on the upper (dorsal) side of the forebrain, between the two cerebral hemispheres. It is about 5-8 millimetres long. The 17th-century French philosopher René Descartes called it the "seat of the soul".

The main hormone secreted by the pineal gland is melatonin. Its secretion is controlled by light and darkness. Light falling on the retina of the eye sends signals, through the hypothalamus, to the pineal gland. In the dark, the pineal gland releases more melatonin, which makes a person feel sleepy. In the light, melatonin secretion is reduced, and the person stays awake. So melatonin plays a key role in regulating the body's 24-hour biological clock, called the circadian rhythm, especially the sleep-wake cycle.

Melatonin also influences body temperature, metabolism, pigmentation, the menstrual cycle and the immune system. Because the pineal gland responds to light, it is sometimes called the "third eye"; in some reptiles and amphibians, the pineal region actually has light-sensitive cells.

Disturbances of the circadian rhythm can cause sleep problems, as in jet lag, which occurs when people travel across several time zones quickly, and in people who work night shifts. Exposure to bright screens, such as mobile phones, late at night can suppress melatonin and make it harder to fall asleep. Melatonin supplements are sometimes used to treat jet lag and insomnia.

In the skin, melanin (a different substance, a pigment) gives colour; it should not be confused with melatonin. Melanocyte-stimulating hormone (MSH) affects skin pigmentation.

Jeffrey Hall, Michael Rosbash and Michael Young won the Nobel Prize in Physiology or Medicine in 2017 for discovering the molecular mechanisms that control circadian rhythms.

Why not the other options?

• ✗The pituitary gland is the master gland, secreting growth hormone and others.
• ✗The thymus secretes thymosin, which helps develop T-lymphocytes for immunity.
• ✗The thyroid gland secretes thyroxine, which controls metabolism.