has subject area Animals Circadian Rhythm Gene Deletion Immunohistochemistry Light Male Mice Mice, Inbred C57BL Mice, Knockout Motor Activity Mutagenesis, Insertional Nitric Oxide Synthase Nitric Oxide Synthase Type I Photic Stimulation Photoperiod Proto-Oncogene Proteins c-fos Suprachiasmatic Nucleus Time Factors