The relationship between the intensity of light pulses and the extent of phase shifts of the circadian rhythm in the eclosion rate of Drosophila pseudoobscura

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Abstract
The advance as well as the dealy phases of the circadian system of Drosophila pseudoobscura respond in a similar manner to exposure to light signals of 15‐minute duration and different intensities. The response, measured by the extent of phase shifts four to five days after the signals, is complete at an intensity of 10 lux; light signals of higher intensities such as 3000 and 10000 lux effect phase shifts of similar time course and magnitude as light signals of 10 lux. Light pulses of intensities below 10 but above 1 lux induce phase shifts of a lesser degree. The response of the system to light signals in this range appears to be a graded one. Low light intensity signals of 0.1, 0.3 and 1.0 lux induce “reversing transients,” i.e., transients having a direction opposite to that seen in the subsequent steady state, whereas light pulses of higher intensities do not induce reversing transients.