Constant Light: Effects on the Circadian Locomotor Rhythm in the House Sparrow

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Abstract
Perch-hopping activity rhythms were studied in house sparrows kept under a variety of environmental lighting conditions with the following results: (1) Circadian rhythms of locomotor activity were well defined in the house sparrow and were entrained by 24-h light-dark cycles. (2) Constant light (≃870 lx) caused 80% of the sparrows to become apparently arrhythmic and constantly active; constant light of lesser intensity (≃240lx) produced arrhythmicity in only 20% of the sparrows. (3) When arrhythmic sparrows from constant light (≃870lx) or several photoperiodic regimens were placed in constant dark their endogenous circadian locomotor activity rhythms were apparent, and the time setting of the rhythms was derived from the time of the lightto-dark transition. The phase of the resultant rhythm was as if the sparrows took the light-to-dark transition as a normal "dusk" (lights-out, onset plus about 12 h). (4) Sparrows entrained to photoperiods of 8 h or more tend to end their activity at the time of lights out. (5) When the phase of a 24-h light-dark cycle was shifted abruptly (by 6 or 12 h) the rate of adaptation to the new cycle was usually faster if the phase shift was made by extending the light rather than the dark period. The experimental results have been interpreted from the point of view of hypotheses that light either stops or masks the clock, and the results have also been related to the role of the pineal gland in sparrow circadian physiology.