These are interesting answers. I never really thought about it. Our bodies do have natural rhythms, and it has been proven that sleep is restorative. If we did not need sleep, why would we sleep? It must perform some function.
I do not believe in biorhythms of any sort. They are just a pseudoscience like astrology. There have been many studies done that have debunked the notion that there are biorhythms, for the skin or for any other part of the body.
Please copy and paste this link to see an essay about that topic.
However, some people do believe in such things. Here is a link supporting the idea.