Circadian Rhythms

We live in a rhythmic world. The rotation of the earth on its axis and the orbit of the earth around the sun give rise to a series of daily and yearly cycles in physical parameters such as light levels and temperature. Life on earth has evolved specific internal timing systems that permit organisms to best exploit these rhythmic and predictable changes in their environment. These systems include a daily circadian timing system that measures internal time on a near 24-hour basis. From an evolutionary perspective, the adaptive advantage conferred on an organism by possessing such a timing system is that it allows an organism to anticipate changes in its environment rather than merely reacting to those changes.

An example of this advantage can be thought ...