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. The 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 related to the cycles. These inherent rhythms are said to control or initiate various biological processes and are classically composed of three cyclic rhythms that are said to govern human behavior and demonstrate innate periodicity in natural physiological change: 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.
Basic rhythms follow certain facets of physiological cycles, though they may include others, and the details may vary depending on the source. The three classical cycles of biorhythms are endogenous infradian rhythms. The theory's basis lies in physiological and emotional cycles. They are often represented graphically as either symmetric or asymmetric waveforms, though most theories rely on symmetric forms. The most commonly used form is the 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 synchronized at their minimum values. According to the classical definition, the theory is assumed to apply only to humans. In the classical theory, the value of each cycle can be calculated at any given time in the life of an individual, and there are web sites that do exactly that.
Biorhythmics is either a protoscientific branch or a pseudoscience, depending on opinion, that studies biorhythms or deal with biorhythms. Biorhythmic study focuses on physiological, emotional, and intellectual processes and its forecasting. Biorhythm phenomena are observable human conditions which can be detailed and explained by biorhythmics. These conditions are bound by the variables that exist in the body. Certain facets of biorhythmics are likened by proponents to concepts found in weather forecasting (commonly known as meteorology).
Chronobiology is a branch of biology that studies rhythms in living beings. Unlike biorhythm, its status as a science is unquestioned.
In the workplace, railroads and airlines have experimented the most with biorhythms. A pilot describes the Japanese and American attitudes towards biorhythms. He acknowledges, researching his pilot logbook, that his greatest errors of judgment occurred during critical days, but concludes that an awareness of one's critical days and paying extra attention is sufficient to ensure safety. A former United Airlines pilot confirmed that United Airlines used biorhythms until the mid-1990s, while the Nippon Express air freight still used biorhythms.
Charting biorhythms for personal use was popular in the United States during the 1970s; many places (especially video arcades and amusement areas) had a biorhythm machine that provided charts upon entry of date of birth. Though biorhythms have declined in popularity, there are numerous websites on the Internet that offer free biorhythm readings. In addition, there exist free and proprietary software programs that offer more advanced charting and analysis capabilities.
The classical theory originated at the turn of the 20th century, between 1897 and 1902, from observational research.
Hermann Swoboda, professor of psychology at the University of Vienna, who was researching periodic variations in fevers, looked into the possibility of a rhythmic change in mood and health. He collected data on reaction to pain, outbreak of fevers, illnesses, asthma, heart attacks, and recurrent dreams. He concluded that there was a 23-day physical cycle and a 28-day emotional cycle.
Wilhelm Fliess, a nose and throat specialist and reportedly a numerologist, was independently researching the occurrences of fevers, recurrent illnesses and deaths in his patients. He too came to the conclusion that there was a 23- and a 28-day rhythm; he labelled the former "male" and the latter "female". His writings on the subject have been described by Martin Gardner as "a masterpiece of Teutonic crackpottery". Fliess was particularly preoccupied with the fact that he could express a variety of numbers related to natural phenomena in the form 23x + 28y by choosing x and y as suitable positive or negative integers. He did not realize that "if any two positive integers that have no common divisor are substituted for 23 and 28 in his basic formula, it is possible to express any positive integer whatever." Fliess' theories were of great interest and importance to Sigmund Freud during his early work in developing his psychoanalytic concepts.
Alfred Teltscher, professor of engineering at the University of Innsbruck, observed that his students' good days and bad days followed a rhythmic pattern of 33 days. Teltscher found that the brain's ability to absorb, mental ability, and alertness ran in 33 day cycles. In the 1920s, Dr. Rexford Hersey (psychologist; Pennsylvania, America) also reportedly made contributions to the classical theory.
These three biorhythms compose the classical theory. The classical theory has been studied, especially in Germany, Japan, and the United States, with conflicting results. Various modern derivatives exist of the classical theory.
Proponents of biorhythmics call it an established interdisciplinary area of scientific endeavor which is still speculative - a protoscience. Critics state that biorhythms are based only upon numerological associations. The plausibility of biorhythmics is contested by mathematicians, biologists and other scientists. The most basic assertion is that even if it is assumed that physiological rhythms do exist, it's not clear why they should necessarily begin on the day of one's birth.
Biorhythmics has echoes of chronobiology, the study of biological rhythms. Through medical research, doctors have found that there are periodic biological cycles in a person's lifespan, such as the circadian rhythm (from Latin circa diem; literally, "about a day"), but few doctors believe they correspond to those described as "biorhythms". To proponents, these discoveries (among others) demonstrate that people are affected by physiological, emotional and intellectual rhythms, though the exact relationships to the biorhythm cycles are not precisely understood. Studies regarding the effects of biorhythms on the human condition are still conducted.
The Biorhythm theory is often treated as falsely claiming scientific validity. Biorhythm critics' responses range from opposing it as harmful to ignoring it or treating it as entertainment. Some of the criticisms of the various theories in the category of biorhythmics are:
Some biorhythm critics say that biorhythms can be thrown off by such occurrences in the calendar as the beginning of the new year, holidays, or something as simple as the start of the week.
There have been some three dozen studies supporting biorhythm theory but all of them have suffered from methodological and statistical errors. An examination of some 134 biorhythm studies found that the theory is not valid.
The equations for the curves are
where t indicates the number of days since birth.