Phase Dependent Changes of Circadian Frequency after Light Steps

Abstract

DIURNAL rhythm has been shown in a great variety of species to be self sustaining, that is, under constant conditions of light and temperature the periodicity continues. The period length of the free running rhythm (τ) generally deviates from the exact 24 h cycle of the solar day; it is then called a “circadian” rhythm. The “circadian rule”¹ states that the period length of circadian rhythms in constant light is a function of the light intensity. The effect of light on the free running rhythm has been regarded as a parametric or proportional influence, that is, a continuous action of light on the biological oscillation^1,2. This interpretation implies that the following period would not be affected by the position of the cyele at which the light intensity is altered.