Biorythm, Who knows that kind of theraphy?

I had this as a highschool project. You can calculate your physical, mental and emotional state through mathematical functions. I don't fully believe in this but there are instances where i've proven it helpful. My professor said there's a certain airline in the states that doesn't let their pilots fly planes when their biorythm is in a critical state.

I do not know what Biorythm is and I never really heard of it either.