This is what I found out:
a biorhythm is a recurring cycle in the physiology or functioning of an organism, such as the daily cycle of sleeping and walking or in other words to properly define the matter a cyclic pattern of physical, emotional and intellectual activity said to occur in the life of a person.
The history of Biorhythms
In the beginning Dr Wilhelm Fliess noticed identical rhythms in the case
histories of his patients. He observed active and passive phases in the
physical, emotional and intellectual/mental aspects of humans. Out of these observations he derived the principle of the Biorhythms: The phsical curve with it's cycle of 23 day, the emotional curve with its cycle of 28 days and the intellectual/mental curve with its cycle of 33 days.
In other words a Biorhythm is one of three cyclic rhythms that are supposed to govern govern behaviour. The theory is usually treated as harmless entertainment, in the sane league as horoscopes and therefore attracts little controversy.
I calculated my Biorhythms on internet by typing the day, month and year of my birth.
My physical cycle is: 82
my emotional cycle is: -90
my intellectual cycle: 69
this is my Biorhythm graph:
(It also calculated my intuition, spiritual, awareness and aesthetic)
I don't really understand how the internet can calculate biorhythms because surely they're how you feel so I created my own biorhythm chart: