The biorhythms are life cycles that start at your birth and last for all your life. They describe very well the energy levels and the capacity for performance on 3 levels: physical, emotional and intellectual.
The physical biorhythm lasts for 23 days and is related to your physical energy, strenght, health, stamina.
The emotional biorhythm last for 28 days (just like the Moon's cycle) and relates to your emotional stability or stress.
The intellectual biorhythm lasts for 33 days and relates to your mental capabilities, creativity, aptitude to solve problems.
Your biorhythm above the horizontal line indicates that your capacity in that area is enhanced; you feel stronger, more alert, more connected, more empathetic. These are times when you are able to do more, be more, enjoy more. When the biorhythm lines are below 0%, your capacity is diminished, and conservative behavior is recommended.
Finally, when a biorhythm line crosses the horizontal line, or crosses another biorhythm line, energies are unpredictable and may not be favorable to you. These are the critical days you must be aware of and NOT schedule any important events that relate to that area.