Biorhythm, a theory that purports to identify periods of increased individual susceptibility to accident or misfortune on the basis of recurring biological cycles, is currently enjoying world-wide popularity. In view of the implications of such a theory for both public health and safety, the present study was undertaken as

an empirical test of its validity. Using data from two hundred and five carefully investigated highway crashes (one hundred thirty-five fatal; seventy nonfatal) in which the drivers were clearly at fault, the authors computed specific points in drivers' biorhythm cycles at which the accidents occurred. The observed frequencies of accidents occurring during so-called critical and minus periods were then compared with the frequencies to be expected on a chance basis alone. The results provided no evidence for a relationship between purported biorhythm cycles and accident likelihood.