What does the pineal gland secrete that regulates sleep-wake cycles?

The pineal gland secretes melatonin, a hormone that plays a crucial role in regulating sleep-wake cycles, also known as circadian rhythms. Melatonin production is influenced by light exposure; it typically increases in response to darkness, signaling the body that it is time to prepare for sleep. As light levels rise, melatonin secretion decreases, helping to wake the body and maintain alertness during the day. This regulation is essential for maintaining a healthy sleep pattern and ensuring that the body's internal clock remains synchronized with the external environment. The pineal gland's ability to produce melatonin makes it a key player in sleep physiology, facilitating the transition between sleep and wakefulness.