The mechanism in the brain responsible for regulating the circadian rhythm is called the biological clock. This mechanism is located in a core area in the hypothalamus called the nucleus suprachiasmaticus. Light striking the retina of the eyes is transmitted via special ganglion cells to the hypothalamus and from there to the suprachiasmatic nucleus. These ganglion cells contain the light-sensitive pigment melanopsin. The suprachiasmatic nucleus regulates the rhythm of wake-sleep by stimulating other brain areas such as the epiphysis (pineal gland). The epiphysis produces the hormone melatonin, which influences the sleep-wake rhythm.