Biorhythm was something very popular in the seventies and eighties. The basic idea is that every human being is subject to emotional/intellectual/physical "cycles", with good and bad phases, following a steady rhythm that begins at birth moment.
This is that kind of superstition that "can't hurt", so it is the kind of thing that one takes a look to decide over undecidable things, and is ashamed of throwing heads or tails. I must admit I look into my own biorhythm from time to time :)