BIOLOGICAL RHYTHMS
A variety of behavioral cycles occur among organisms. These biological rhythms orient the organism to environmental cues such as tides, solar cycles, and annual patterns. Circadian rhythms are daily activities, such as lizards feeding during the daytime. Diurnal animals, including most birds, are active during the daytime. Nocturnal animals, such as the owl monkey, are active at night. Crepuscular animals do most activities at dusk and dawn.

These rhythms are controlled by external stimuli (such as the lunar cycle) and by internal biological clocks. The suprachiasmatic nucleus (SCN) in the hypothalamus is the home of the mammalian master clock. The pineal gland and hormones also play a part.

Melatonin is an endocrine hormone that may be involved with human biological clocks. How might you go about analyzing the value of melatonin in controlling your own sleep patterns?