In Syrian and European hamsters, the duration of sensitive phase to light of the suprachiasmatic nuclei depends on the photoperiod.

Light induction of the expression of Fos protein in the suprachiasmatic nuclei was used to investigate the photosensitive state of the clock in Syrian and European hamster kept under different photoperiods. We observed that the duration of the photosensitive phase is variable and tied to the length of the night. A maximal extension has been determined in both species studied. Finally, a 4 h lengthening of the phase of photosensitivity take approximately 3 weeks, while 3 days only are needed for its shortening.