Circadian rhythms are what people often refer to as your body's internal, biological clock. The typical human circadian rhythm occurs on a cycle of approximately 24 hours. However, the clock is not really functioning on time, but on body temperature. It is just that body temperature fluctuates on somewhat of a regular type of schedule, and so many people often believe that the circadian rhythms are time oriented instead of body temperature oriented. For example, your body temperature begins to increase in the morning (as you wake and start your day), then gets higher during the day while you are active, and begins to drop during the evening, producing feelings of fatigue and preparing for sleep.