All, I hope this describes more about them than I can explain see link: Wikipedia on Biorhythms

Back in the 1970's-80's it was a big thing when I lived in California back then.

Also, if you go to the link on my wall, enter your own birth data and calculate it you will see your own chart. Double click on the yellow highlighted section for today and it'll zoom in on that timeframe. At the bottom of the page you will see a synopsis of what biorythms are that describes it better than the Wikipedia link.

It starts like this:
In the nineteenth century, studies first began on certain life rhythms or cycles that were later termed "biorhythms". The word biorhythm is a compound of two Greek words, bios and rhythmos, which mean life and a constant or periodic beat. The theory of biorhythms defines and measures three basic and important life cycles in man: the physical, emotional, and intellectual.

Wilhelm Fliess, a highly respected and prominent doctor in Berlin, did pioneer work on biorhythms in the 1890s. Fliess, who had observed 23- and 28-day rhythms in many of his patients, began to collect statistics on the periodic occurrence of fevers, childhood disease, and the susceptibility to disease and death. With these statistics in hand, Fliess believed he had detected rhythms which were fundamental to man's life.

Dr. Fliess later developed two major biorhythm theories: first, that Nature bestows on man "master internal clocks" which begin counting time at birth and continue throughout life; and second, that one of these clocks regulates a 9-3-day cycle influencing man's physical condition and another regulates a 28-day cycle influencing emotions or degree of sensitivity.