Biorhythm theory was born in the late 19th century in the fevered imagination of Wilhelm Fliess, a Berlin physician and close friend of Sigmund Freud, who became convinced that human life pulsed in hidden 23- and 28-day cycles — which he helpfully labeled "male" and "female." A Viennese professor named Hermann Swoboda independently reached the same conclusions in 1904, prompting Fliess to accuse him of intellectual theft, which is a very dramatic thing to fight about when you're both wrong. The 33-day intellectual cycle was added in the 1920s by an engineering professor who noticed his students had good days and bad days, a phenomenon that has since been attributed to coffee.
Biorhythm theory holds that from the moment of your birth, three invisible sine waves begin oscillating through your body — a 23-day Physical cycle, a 28-day Emotional cycle, and a 33-day Intellectual cycle — governing your strength, mood, and brainpower with the reliability of a Swiss watch and the scientific validity of a horoscope. The theory was popularized in the 1970s by a man named Bernard Gittelson, who sold millions of books and pocket calculators to people eager to know whether Tuesday was a good day to sign a mortgage.
Controlled studies have repeatedly found that your biorhythm score predicts your performance about as well as a coin flip, and that "critical days" — when a cycle crosses zero — are no more dangerous than any other day, which will come as a disappointment to the Japanese companies that once scheduled employee shifts around them. None of this stopped the theory from briefly captivating the sports world, the business world, and at least three US presidents' scheduling staff.
Enter your birthday above and enjoy your completely meaningless results.
In 1982 I was President of the Science Club (I know) and I charged .25 for a Biorythm. In 2026 I paid Claude .24 cents of API time (10 minutes/40K tokens) and built this. Who's the president now.