What hormone is primarily produced by the pineal gland and regulates sleep-wake cycles?

The hormone produced primarily by the pineal gland that regulates sleep-wake cycles is melatonin. The pineal gland, located in the brain, secretes melatonin in response to darkness. This hormone plays a crucial role in signaling the body that it is time to sleep. Melatonin levels typically rise in the evening as light decreases, promoting sleepiness, and decrease in the morning when light is present, helping to wake the body up.

Understanding the role of melatonin in regulating circadian rhythms is essential in fields related to sleep disorders, behavioral science, and overall health. Other hormones like serotonin, cortisol, and adrenaline have different functions in the body. Serotonin, for instance, is primarily involved in mood regulation and is not produced by the pineal gland. Cortisol, the stress hormone, follows a diurnal rhythm but is synthesized in the adrenal glands, while adrenaline (or epinephrine) plays a key role in the body's fight-or-flight response, rather than regulating sleep.