Which hormone, produced by the pineal gland, is responsible for regulating circadian rhythms?

The hormone produced by the pineal gland that regulates circadian rhythms is melatonin. Melatonin plays a crucial role in signaling the body to prepare for sleep as it is released in response to darkness, helping to synchronize the sleep-wake cycle with the day-night cycle. This signaling is part of the broader circadian rhythms, which govern various physiological processes over a 24-hour period.

While cortisol, adrenaline, and serotonin are important in various bodily functions, they do not primarily regulate circadian rhythms. Cortisol, for instance, is released in response to stress and has a diurnal pattern of release but is not directly involved in the regulation of sleep and the circadian clock. Adrenaline is associated with the body's fight-or-flight response. Serotonin is a neurotransmitter that contributes to mood regulation and is occasionally converted to melatonin, but it does not inherently manage circadian rhythms. Thus, melatonin is specifically recognized for its function in modulating sleep patterns and circadian cycles.