The pineal gland is a small, pea-shaped endocrine gland located deep within the brain. It is primarily responsible for producing the hormone melatonin, which helps regulate sleep-wake cycles and circadian rhythms. The gland is sensitive to light, meaning its activity is influenced by the amount of light exposure, which is why melatonin levels typically rise in the evening and fall in the morning. In addition to its role in sleep regulation, the pineal gland has been studied for its potential effects on mood and seasonal affective disorder (SAD). While its exact functions are still being researched, the pineal gland is considered an important part of the body's hormonal system.