The pattern of human physiological processes that seems to be controlled by an internal "biological clock" is

They are sensitive to daylight-dark cycles. Biological clocks drive or alter patterns, alertness, mood, physical strength, blood pressure and every other aspect of our physiology. Our biological clocks drive our circadian rhythms. These internal clocks are groupings of interacting molecules in cells throughout the body. A "master clock' in the brain coordinates all the body clocks so that they are in synch. 

Oh dog