What is the relationship of melatonin to circadian rhythms?

Melatonin plays a crucial role in the regulation of circadian rhythms, which are the biological processes that display an endogenous, entrainable oscillation of about 24 hours. The primary function of melatonin is to signal the body that it is night, thereby helping to regulate sleep-wake cycles.

Melatonin is synthesized by the pineal gland, with its production being tightly regulated by light exposure; levels typically rise in the evening as it gets dark, peak during the night, and decrease in the morning when it becomes light. This pattern of secretion helps to synchronize various biological processes with the external environment, particularly by promoting sleepiness in the evening and facilitating wakefulness during the day.

The other choices do not accurately reflect the relationship of melatonin to circadian rhythms. Melatonin is not predominantly secreted during the day; rather, it is secreted mainly at night. While it is associated with the hypothalamus in terms of sleep regulation, it is not produced exclusively there, nor does it peak in the afternoon. Understanding melatonin's role in modulating circadian rhythms is essential for grasping how the body maintains a regular sleep-wake cycle and responds to environmental light changes.