Jet lag is where a person is used to the daily cycles of one place and then moves quickly to another place before the body has a chance to adjust to new daily cycle. The human biorhythm theory was formed in the 1800's. This theory advocates that everyone functions on three basic life cycles called biorhythms. These cycles start on the day a person is born. The three basic cycles are physical, emotional, and intellectual.
Some people add a fourth called intuitive.
Each biorhythm has three phases, a high phase, a low phase and a critical phase. The easiest was to view this cycle is as a wave form, know as a sine wave. These waves continue for the rest of our lives. The Physical wave is a cycle of 23 days. It controls our energy, endurance, initiative, stamina, vigor and vitality.