Photoperiodism and Circadian Rhythms: An Hypothesis

Abstract

As might be expected living organisms have adapted to monitor variations in day length and thereby anticipate forthcoming seasonal changes. In discussing day length we usually refer to the day as that portion of the 24-hour period from sunrise to sunset. In connection with photoperiodism (the response to day length) the illuminated portion of the 24-hour cycle is referred to as the photoperiod. Since a solar day consists of a photoperiod and a dark period and since the solar day is constant, it is obvious that any change in the day length will produce a change in two variables, the photoperiod and the dark period, which vary inversely one to the other. Therefore, an organism can measure day length by measuring either the length of the photoperiod or the length of the dark period, or it might determine day length by the ratio of these two. Since photoperiods increase day by day throughout half of the year and then decrease day by day through the second half of the year, it also seems possible that the rate of change in day length and the direction of such change may be factors in determining or timing physiological responses. Throughout the course of organic evolution organisms have been exposed to cyclic changes in day length year after year. It would be surprising, therefore, if organisms had not evolved (via natural selection) a wide variety of mechanisms that measure day length. Furthermore, a given species may contain the basic genetic information for the utilization of all the methods of measurement.