Biorhythm is a theorie of those who think that our lives are regulated by cycles. Our life and our body in one way.
Thrre cylces are leading our lives: physical, intellectual and emotional.
Biorhythm is not a psychic reading technic, it is just like astrology can be, a manner to understand ourselves. It is a way to hear ourselves, it could be very instructive.
In fact there are no evidence that biorhythm exist and can be true, that's why we can find that in esoterism website or books, but not in science.
How works biorhythm?
Each cycle of biorhythm oscillates between a positive and a negative phase, during which bioelectric activity strengthens and weakens. There are waveforms, they start from zero percent when we birth and continue there movement during the life. Each day waveform are moving crossing a line, a line which represent a kind of basis or basic point. At the end the goal biorhythm is to map and explain these rythms.
It is not very easy, there are calcul methods but they can be differents between some calculators.
The problem with biorhytm is hard: this is not a science and not a psychic reading method, so what it is? Is it hard to answer.
In fact all scientists regects that theorie, and many fortune tellers prefer using astrology and other psychic reading methods in order to obtain responses than using biorhythm theorie...