Three-dimensional visualization based on segmented human data of the pineal gland (yellow), a small organ located on the posterior part of the roof of the third ventricle, seen here below the corpus collosum. It is connected to the brain via a short stalk containing nerve fibers which communicate with the hypothalamus. The pineal gland secretes the hormone melatonin which regulates the circadian rhythms of the body. It's secretion during hours of darkness produces a hypnotic effect which results in sleep.