The Three Cycles
Biorhythm theory holds that three cycles begin at birth and run unbroken for life, each a simple sine wave through a fixed period.
Level = sin(2π × days since birth ÷ cycle length) × 100
| Cycle | Length | Said to govern |
|---|---|---|
| Physical | 23 days | Strength, stamina, coordination, resistance to illness |
| Emotional | 28 days | Mood, sensitivity, creativity, sociability |
| Intellectual | 33 days | Alertness, memory, analytical reasoning |
A cycle above zero is called a high phase, below zero a low phase. The day a cycle crosses zero is a critical day — treated in the theory as unstable rather than simply bad. Because the three periods share no common factor other than one, all three cross together only once every 21,252 days, or roughly every 58 years and two months.
Reading the Chart
- Peak (+100) — the top of a cycle, reached a quarter of the way through it.
- Rising phase — the theory's preferred time for demanding work in that domain.
- Critical day (0) — a crossing point. Each cycle has two per period, so there are roughly six critical days a month across the three.
- Trough (−100) — described as a recovery phase, not a deficit.
Where the Idea Came From
Wilhelm Fliess, a Berlin ear-nose-and-throat surgeon and a close correspondent of Freud, proposed 23- and 28-day cycles in the 1890s. The 33-day intellectual cycle was added by Alfred Teltscher, an Austrian engineering teacher, in the 1920s. The idea reached a wide audience in 1970s America, when pocket biorhythm calculators sold in large numbers and a handful of companies briefly scheduled shifts around them.
Does It Work?
No. This is one of the more thoroughly tested claims in popular psychology, and the result is consistent.
- A 1978 review in Psychological Reports examined more than a hundred studies and found no support for the theory.
- Terence Hines' 1998 review in the Skeptical Inquirer reached the same conclusion across accident rates, athletic performance and academic results.
- Analyses of industrial accident data, once corrected for the base rate of critical days, show no clustering at all.
- The cycle lengths were fixed before any data existed and have never been revised, which is the signature of numerology rather than of a measured phenomenon.
Genuine human rhythms do exist — the roughly 24-hour circadian cycle, the menstrual cycle, seasonal variation in mood — but they are entrained by light, hormones and behaviour, they vary between people, and they drift. A fixed 23-day wave running unaltered from birth to death is not among them.
Compatibility Charts
A common variation compares two people by measuring how closely their cycles align, expressed as a percentage. It is arithmetic on the difference between two birth dates and carries exactly as much predictive weight as the single-person chart, which is to say none. It remains a pleasant enough thing to look at.
Frequently Asked Questions
What is a critical day?
A day on which a cycle crosses the zero line. The theory treats these as days for caution rather than as unlucky days. There are about six a month.
Why 23, 28 and 33?
Fliess chose 23 and 28 from a numerological scheme in which he believed those numbers underlay all biological periodicity; 33 was appended decades later by a different author. None of the three was derived from measurement.
Can biorhythms predict a good day for an exam or a race?
No. Sleep, preparation, nutrition and stress account for the variation people attribute to these curves, and all four are worth attending to instead.
Do all three cycles ever peak on the same day?
Only at the shared multiple of the three periods: 23 × 28 × 33 = 21,252 days, or about 58 years and two months after birth — and then again shortly before the 117th birthday.