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 int ensity with maximum advance (at circadian time 20 [CT20]) and maximum delay phase-shifts (at CT15). The phase-shifts evoked by natural daylight stimul i of various illuminations ranging between 0.001 lux and 10,000 lux and las ting 15 min were estimated. The results clearly demonstrate that the relati onship between the phase-shifts and the intensities of Light stimuli is non linear. Furthermore, a single light stimulus of 0.001 lux, or 0.1 lux inten sity for a duration of 15 min, administered at CT20, evoked unequivocal res ponses; phase delays were observed instead of phase advances. The critical intensities needed for light stimuli of 15 min duration to induce saturatin g 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 l ight is required at the phase CT15 to induce a saturating phase shift than is required at a later phase of the circadian cycle (CT20). J. Exp. Zool. 2 83:181-185, 1999. (C) 1999 Wiley-Liss,Inc.