Final answer:
The pineal gland is an endocrine structure that produces and secretes melatonin, regulating the sleep-wake cycle and circadian rhythms. It is located in the brain and is influenced by light levels. Jet lag can affect the pineal gland's function.
Step-by-step explanation:
The pineal gland is an endocrine structure of the diencephalon of the brain, located inferior and posterior to the thalamus. It is made up of pinealocytes that produce and secrete the hormone melatonin. Melatonin is produced in response to low light levels and regulates the sleep-wake cycle. High levels of melatonin induce drowsiness. The pineal gland is also involved in circadian rhythms and can be affected by jet lag when traveling across time zones.