A biorhythm  is a hypothetical cycle in physiological, emotional, or intellectual well-being or prowess. "Bio" pertains to life and "rhythm" pertains to the flow with regular movement. Biorhythms theory has no more predictive power than chance, and has been labeled a pseudoscience by skeptics

The theory of biorhythms claims that one's life is affected by rhythmic biological cycles, and seeks to make predictions regarding these cycles and the personal ease of carrying out tasks. These inherent rhythms are said to control or initiate various biological processes and are classically composed of three cyclic rhythms :the physical, the emotional, and the intellectual (or mental) cycles. Others claim there are additional rhythms, some of which may be combinations of the three primary cycles. Some proponents think that biorhythms may be potentially related to bioelectricity and its interactions in the body.

The theory's basis lies in physiological and emotional cycles. They are often represented graphically as sinusoidal waveform, which is thought to be a plausible representation of a bioelectric activity cycle. Due to this sinusoidal nature, the cyclical flow of bioelectric activity undergoes periodic reverses in direction. Each cycle oscillates between a positive phase [0%..100%] and a negative phase [-100%..0%], during which bioelectric activity strengthens and weakens.

The waveforms start, in most theories, at the neutral baseline (0%) at the time of birth of each individual. Each day that the waveform again crosses this baseline is dubbed a critical day, which means that tasks in the domain of the cycle are far more erratic than on other non-critical days. The purpose of mapping the biorhythmic cycles is to enable the calculation of critical days for performing or avoiding various activities.

The classical definition (derivatives of the original theory exist) states that one's birth is an unfavorable circumstantial event, as is the day about 58 years later when the three cycles are again synchronised at their minimum values. According to the classical definition, the theory is assumed to apply only to humans.  The value of each cycle can be calculated at any given time in the life of an individual, if the date of birth is known and there are web sites that do exactly that.

----wikepedia