References on Light and the Biological Clock

Seasonal Variations in Body Weight and Metabolism in Hamsters. In Seasonal Affective Disorders and Phototherapy 1989; 105-126. N.E. Rosenthal and M.C. Blehar Eds. Guilford Press New York, London
G.N. Wade.

"Temperate-zone mammals often exhibit dramatic fluctuations in a wide variety of behaviors and physiological processes, including reproduction, body growth and fat storage, thermoregulation, and metabolic and endocrine functions. These changes allow the animal to adjust their activities, particularly production of offspring, in anticipation of seasonal variations in food supplies and metabolic demands. The ability to anticipate these climactic changes, rather than simply to react to them, would seem to be advantageous."

A "Clock For All Seasons" in the Human Brain. Progress in Brain Research 1996; 111:321-42
T.A. Wehr.

"Because the earth's axis of rotation is tilted the durations of daily periods of darkness and light to which organisms are exposed vary systematically during the course of the year. In many animals, the circadian pacemaker is able to take this variation into account by detecting seasonal changes in day length and making corresponding adjustments of the biological day and night within. This in the winter when the nights are long, the biological night is long. In summer when nights are short the biological night is short. This led Pittendrigh and Daan to observe in their classic description of the formal properties of the circadian pacemaker, that it is a "clock for all seasons" ...There is now considerable evidence that the foregoing description applies to the human circadian system too. " Abstract