Many things work in cycles. Everyone is familiar with a day. A day is the cycle of the Earth rotating on it's axis in regards to it's rotation around the Sun.
Many travelers know the feeling of jet lag.
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.