Daylight and Artificial Light Phase Response Curves for the Circadian Rhythm in Locomotor Activity of the Field Mouse Mus booduga

The sensitivity of the circadian pacemaker of the field mouse Mus booduga to diffuse daylight pulses of 15 min duration and 1000 lux intensity was measured in a series of experiments and a phase response curve (PRC) constructed. The PRC evoked was of type-I with significant quantitative differences compared to the PRCs constructed for other light stimuli (fluorescent and incandescent light sources) of the same intensity and duration. Not only are the advance and delay peaks larger in the daylight PRC, but there is also significant qualitative and quantitative differences in the shape of the PRC. The area under the advance zone of the diffuse daylight PRC (A) was found to be signifi-cantly greater than the A of (i) the fluorescent light PRC and of (ii) the incandescent light PRC. The area under the delay zone of the diffuse daylight PRC (D) was also found to be significantly greater compared to D of (i) the fluorescent light PRC and of (ii) the incandescent light PRC. These differ-ences in the shape of the PRCs can only be attributed to the variation in the spectral intensity distribution of the different components in the three light stimuli used since all other factors (e.g. sample size, free-running period, duration and intensity of light pulse etc.) were comparable in the three PRC experiments. We conclude that the circadian photoreceptor(s) of the field mouse M. booduga are sensitive to variations in the spectral composition of light stimuli.