Our Tendency to Feel Wakeful and Sleepy on a 24-Hour

Our tendency to feel wakeful and sleepy on a 24-hour basis depends mainly on

A) a cycle generated by a mechanism in the brain.
B) our perception of environmental stimuli such as heat and light.
C) a cycle generated by hormone-producing organs such as the adrenal gland.
D) wear and tear on the muscles followed by a period of repair.