Diurnal organisms are those that are active mainly during the day. When activity patterns of an organism occur in cycles of about twenty-four hours, the pattern is called a circadian rhythm. Diurnal animals, which sleep during the night and are active during the day, or, conversely, nocturnal animals, which sleep during the day and are active at night, follow a circadian rhythm. Scientists believe that circadian rhythms are controlled by an internal timing mechanism called a biological clock. The exact nature of this internal timing is not known, but varying levels of hormones are thought to play a role.