Chronobiology (Circadian Rhythms)

Abstract

Our earth is spinning, making a complete turn in 24 h. If the axis of this rotation were in the plane of the sun’s rays, half the earth would always be in darkness, half in light. Fortunately for us, the earth does not spin in this manner. Instead, its axis of rotation is roughly perpendicular to the sun’s rays, so that most of the surface of the earth is alternately illuminated and in shadow. All organisms have evolved in this changing illumination, which we know as day and night. It is not surprising, then, that they have adapted to this situation. For example, some animals are active only during the day, while others are nocturnal. Perhaps only the bacteria disregard the periodic quality of natural illumination.