You got to be in shape. Football is an emotional game. The passion to win must be strong. These cycles are not altered or effected by external stresses. Many biological functions are rhythmic such as heartbeat and breathing.
Longer patterns are common such as monthly menstrual cycles and annual seasonal cycles.
The daily rise and fall of blood pressure, excretions and changes in body temperature are well known and documented. Some of these structures have been identified in several species as groups of neurons able to generate cycles. Neuroscience has developed many methods to study biological rhythms and clocks. These include sophisticated computer programs and statistical analyses to discover the rhythmical components of organic and behavioral activities. More and more, chronobiology is discovering how, when and why our brains and hormones are modulated by nature's cycles.