A person's life is influenced by rhythmic biological cycles that affect one's ability in various domains, such as mental, physical and emotional activity. These cycles begin at birth and oscillate in a steady fashion throughout life; thus, by modeling them mathematically, a person's level of ability in each of these domains can be predicted from day to day. Sinusoidal behavior would be expected from chemical and hormonal secretion functions with their biofeedback activity with respect to chemical concentration.