Relationship between light intensity and phase resetting in a mammalian circadian system

The light-induced phase-resetting response of the locomotor activity rhythm in the field mouse Mus booduga was studied at two phases of the circadian cycle known to respond to light stimuli of 15 min duration and 1000 lux intensity with maximum advance (at circadian time 20 [CT20]) and maximum delay phase-shifts (at CT15). The phase-shifts evoked by natural daylight stimuli of various illuminations ranging between 0.001 lux and 10,000 lux and lasting 15 min were estimated. The results clearly demonstrate that the relationship between the phase-shifts and the intensities of light stimuli is nonlinear. Furthermore, a single light stimulus of 0.001 lux, or 0.1 lux intensity for a duration of 15 min, administered at CT20, evoked unequivocal responses; phase delays were observed instead of phase advances. The critical intensities needed for light stimuli of 15 min duration to induce saturating response were calculated and were found to be about 100 lux for CT20 and about 500 lux for CT15. These results suggest that a greater intensity of light is required at the phase CT15 to induce a saturating phase shift than is required at a later phase of the circadian cycle (CT20).