What hormone is produced by the pineal gland?

The hormone produced by the pineal gland is melatonin, which is crucial for regulating sleep-wake cycles. The pineal gland is located in the brain and is responsible for converting serotonin, a neurotransmitter derived from the amino acid tryptophan, into melatonin. This conversion predominantly occurs in response to darkness, signaling to the body that it is time for sleep.

Melatonin plays a key role in maintaining circadian rhythms, which are the physical, mental, and behavioral changes that follow a daily cycle. These rhythms are influenced by environmental factors such as light and darkness. Levels of melatonin increase in the evening and decrease in the morning, helping to regulate the body's internal clock.

Other hormones listed have different functions: serotonin is involved in mood regulation and is a precursor to melatonin, dopamine plays a role in reward and pleasure systems, and cortisol is known as the stress hormone that is involved in various metabolic functions and stress responses. However, none of these hormones are synthesized by the pineal gland, making melatonin the only correct answer in the context of this question.