Relation of Behavioural Activity Rhythms To the Changes of Day and Night. a Revision of Views

All free-living animals have their lives divided into periods of rest and of activity, and these periods are tied to the changes of ambient factors recurring through the diel. These changes are essentially caused by the changing altitude of the sun which divides the diel into day and night with all the associated phenomena. Of these variable factors, the illumination is by far the most important one. With very few exceptions, all free-living animals are exposed every single diel to the change from light to darkness; a change not only of enormous amplitude but also, and perhaps still more important, of inexorable regularity. Temperature, humidity, and wind fluctuate too in accordance with the sun's altitude, but they depend also on other more complex factors such as displacement of airmasses, and are therefore, of secondary importance. The problem dealt with below is: By which mechanism does the changing altitude of the sun, that is the hours of the diel, determine the periodicity of behavioural activity through these factors.