The physiological functions of the pineal are unclear. It secretes the hormone melatonin, which appears to be associated with circadian rhythms and setting the biological clock. Melatonin is mostly released at night when it is dark and its secretion diminishes during the day when it is light. The release of melatonin can be influenced by artifically mimicking day and night, such as with indoor lighting; also, its effects on sleep explain why it is used in some medications to induce sleep.