The Cincinnati area skeptics group (The Association for Rational Thought) has a project of having its members write "skeptical blurbs" - short pieces with bibliographies that quickly give a skeptical view of a fringe topic, often useful as something quick to send to the media when they cover one of these topics. Below is one written by Andrew Lutes. If any REALL members are interested in writing a "blurb" about a subject they are knowledgeable in, please feel free to send it in!
The term "biorhythms" refers to a system of cycles in human behavior developed by one Wilhelm Fliess, a contemporary of Sigmund Freud. The idea is that human behavior is strongly influenced by inflexible 23-day (physical), 28-day (emotional), and 33-day (intellectual) cycles that begin at birth, regardless of your health and other circumstances. There are supposedly high and low points in these cycles. Graphs can be developed to chart these cycles and indicate when supposedly one's best and worst days will be.
Unfortunately, the whole system does not work. There is no evidence to suggest that human behavior runs on inflexible 23, 28, and 33 day cycles. In tests, people do just as well with fake biorhythm charts as real ones. Either the supposed critical days on their real charts were ordinary physical and mental ones for them, or believing that days on fake charts were critical ones made people act like they were. Graphs based on the birth dates of athletes show no correlation with their actual performance -- they do as well on supposedly bad days as supposedly peak ones. Biorhythms bear a strong resemblance to numerology and astrology.