The biorhythm concept revisited: A narrative critical review and epistemological analysis of a persisting but scientifically invalid term in chronobiology

The term biorhythm, introduced in the early 20th century by Wilhelm Fliess, a German otolaryngologist and associate of Sigmund Freud, refers to the supposed existence of universal and deterministic biological cycles governing human physical, emotional, and intellectual capacities. Fliess proposed that human behavior and physical performance follow fixed 23-day (physical), 28-day (emotional), and 33-day (intellectual) cycles - sinusoidal and predetermined from birth. Despite the lack of experimental validation and the absence of any identified physiological mechanisms, this term continues to appear in contemporary scientific literature, where it is often used imprecisely to designate validated biological rhythms such as circadian variations. This article constitutes a narrative critical review with historical and epistemological analysis rather than a systematic review conducted under PRISMA guidelines. It analyzes the historical origin of the concept of biorhythm, the reasons for its lexical persistence, and the epistemological risks it poses to scientific rigor. The review aims to clarify the distinction between the pseudoscientific concept of biorhythms and scientifically validated biological rhythms studied in modern chronobiology. By comparing this concept with the biological rhythms recognized by modern chronobiology - circadian, ultradian, and infradian - which have measurable physiological correlates, and by examining its use in interpreting the biological effects of environmental factors, we highlight how scientifically inappropriate the term biorhythm is. Modern chronobiology provides a mechanistic, measurable, and falsifiable framework for studying physiological rhythms and their environmental modulation. Beyond its historical inadequacy, the continued use of the term also generates terminological redundancy and conceptual ambiguity, since validated chronobiological terminology already offers precise and operational descriptors of biological periodicity. The abandonment of the term "biorhythm" in the scientific literature appears necessary to preserve conceptual clarity and methodological validity, particularly in a cross-disciplinary field such as chronobiology.