Which gland is part of the endocrine system and plays a major role in sexual development, sleep, and metabolism?

The pineal gland is a crucial part of the endocrine system, primarily responsible for the production of melatonin, a hormone that regulates sleep-wake cycles. This regulation of sleep can significantly impact sexual development and metabolism, as these processes are influenced by circadian rhythms. Melatonin secretion is particularly responsive to light exposure, with levels increasing at night to promote sleep, which in turn contributes to the body's overall hormonal balance and metabolic functions.

While other glands listed, such as the thyroid and adrenal glands, are involved in metabolism and can influence aspects of sexual development, they do not have the same direct connection to the sleep cycle that the pineal gland does. The pancreas is more associated with insulin production and blood sugar regulation rather than the broader roles in sexual development, sleep, and metabolic processes that the pineal gland encompasses.