The pineal gland is a pea-sized gland that apparently regulates activity levels over the course of a day. Increased levels of light in the morning stimulate the pineal gland, which in turn reduces the amount of the hormone melatonin it releases into the bloodstream. As a result, body temperature rises, and the organism becomes more active – it “wakes up” and prepares for a new day. At the end of the day, as light levels decrease, the pineal gland releases more melatonin, which lowers body temperature and reduces the organisms overall level of activity in preparation for sleep. These effects are most evident in lower animals such as birds and rats; in fact, in birds sunlight can shine right through the skull onto the pineal gland. It is possible that the pineal gland has similar effects on humans, though research evidence is not yet available at this point. There is some speculation that people who suffer from seasonal affective disorder – that is, who become depressed during the dark winter months – may be suffering from too much melatonin released by the pineal gland in response to the reduced light in that season, though research on this hypothesis is only in its early stages.

Reference:

1. Understanding Psychology: 3^rd Edition. Charles G. Morris.