Final answer:
The pineal gland secretes the hormone melatonin, which is vital for regulating the body's circadian rhythm and sleep-wake cycles. It responds to light and darkness, affecting melatonin levels and thereby influencing sleep patterns.
Step-by-step explanation:
The hormone melatonin, which helps regulate the body's circadian rhythm, is secreted by the pineal gland. This gland is an endocrine structure located within the brain, specifically inferior and posterior to the thalamus. Melatonin production is stimulated by darkness and inhibited by light, playing a critical role in sleep-wake cycles and other biological rhythms. When the retina detects light, this information is conveyed to the suprachiasmatic nucleus (SCN) and then to the spinal cord, ultimately reaching the pineal gland where it affects melatonin secretion. Conversely, as light levels decrease, melatonin production ramps up, leading to an increase in drowsiness.