The pineal gland is considered part of the epithalamus (epi=upon the thalamus) and receives stimuli from the hypothalamus. The pineal gland secretes melatonin during the dark periods. This establishes our biological clock and regulates our circadian rhythm (day-night cycle) which affects many behaviors such as sleeping, eating, sexual desire, etc. Individuals who receive insufficient light, such as those living in the far north during the winter, may experience Seasonal Affective Disorder. This disorder affects their mood and mental state as well as their physiology. Lights which stimulate the retina in a pattern similar to that of normal sunlight has been shown to alleviate the problem.