The Biorhythm theory
  --------------------
  The biorhythm theory is mainly based on a large statistical material.
  There are three classical curves in the biorhythm calendar: the physical,
  the psychical (or emotional), and the intellectual. The duration of a
  cycle of a curve is 23, 28 or 33 days, respectively. When these curves are
  above the middle line (i.e. to the right on the graph), you have extra
  energy, and when they are "down" i.e. to the left on the graph), you have
  low energy in the areas in question. When a curve crosses the midline, you
  call it a "critical day", where you may be instable and out of balance in
  the respective fields (physically, emotionally or intellectually). If
  several curves crosses the midline simultaneously, you have to be extra
  careful and take your precautions.

  If two curves cross each other below the midline (cross combinations),
  it is not as serious as a critical day, but has some influence.

  A biorhythmical "day and night" starts 12 hours before you birth time of
  day, and ends 12 hours after that hour.

  Please see the litterature for further elaboration of the subject:

  Litterature
  -----------
  Hugo Max Gross  : Biorhythmik. Hermann Bauer Verlag. 1959. 6. opl.
                    1979. (German).
  Barbara O'Neil  : Biorhythms: How to Live with Your Life Cycles.
                    New York 1976.
  Arbie Dale      : Biorhythm. New York 1976.