Biorhythm

Where it came from

In the 1890s Berlin doctor Wilhelm Fliess, a friend of Sigmund Freud, proposed that we all run on a 23-day physical cycle and a 28-day emotional one. Vienna psychologist Hermann Swoboda reached similar ideas around 1904, and in the 1920s Alfred Teltscher, an engineering professor in Innsbruck, added a 33-day intellectual cycle: he believed his students’ good and bad days followed that rhythm.

Biorhythms became a craze in the 1970s: books, pocket calculators and arcade machines printed charts, and researchers set out to test whether “critical days” really brought more accidents.

How it’s worked out

Each cycle is a wave that starts at the middle line, rising, on the day you were born: sin(2π × days since birth ÷ cycle length). A critical day is the day a cycle crosses the middle line. This calculator uses those classic rules exactly; there’s no newer or “more accurate” version, because the cycle lengths are fixed by the theory itself.

Was it ever tested?

Many times. A 1998 review by psychologist Terence Hines looked at 134 studies: 99 found nothing, and the 35 that claimed an effect had errors in their methods or math. Large careful tests, like one of more than 2,400 Army aviation accidents, found no link to critical days. A few small recent factory-safety studies report links, but each blames a different cycle, which is what chance tends to look like.

So enjoy it as a curiosity and a bit of history, not a forecast.

Sources: Wikipedia: Biorhythm · Hines (1998), Psychological Reports 83:19–64 · Army aviation accident study · reaction-time study