The human body naturally has several patterns that recur periodically. Without the art of biorhythms, studying the rhythms of the body is fairly useless; the body’s ecosystem is fairly delicate, and the conscious mind can overcome and equally, subvert, natural variation.
The easiest of the cycles to access magically is the intuitive, because it’s the most responsive to the trance states common in magic. To get in touch with it without magic, spend time doing some mindless work, and alternate that with meditation. This promotes the right mental habits to access the intuitive rhythm.
Accessing the other biorhythms is beyond the scope of an introductory post.