According to believers in biorhythms, a person's life would be determined by rhythmic biological cycles that would affect the capacity of each individual in different fields, such as the mental, the physical or that of the emotions. These cycles would start at birth and oscillate according to a sine wave throughout life. In this way, the capacity of a person in each of these terrains could be predicted day by day using an ad hoc mathematical model.
Most models that are based on biorhythms define 3 cycles: a 23-day "physical" cycle, a 28-day "emotional" cycle, and a 33-day "intellectual" cycle. Although the 28-day cycle would last as long as the average menstrual cycle for women and would in principle have been termed a "female" cycle (see below), both cycles would not necessarily be synchronized. Each of these cycles would vary sinusoidally between two extremes, high and low. The days when the cycle crossed the zero axis would constitute a sort of "critical day" of increased risk or uncertainty.
In addition to the 3 cycles that are better known, other cycles have been proposed based on the linear combination of the first 3 or on oscillation rates that are either shorter or longer.