title: Twilight and photoperiod affect behavioral entrainment in the house mouse (Mus musculus).
abstract: The effect of twilight transitions on entrainment of C57BL/6JOlaHsd mice (Mus musculus) was studied using light-dark cycles of different photoperiods (6, 12, and 18 h) and twilight transitions of different durations (0, 1, and 2 h). Phase angle differences of the onset, center of gravity, and offset of activity, activity duration (alpha), as well as free-running period (tau) in continuous darkness were analyzed. The main finding was that for all conditions the onset of activity was close to dusk or lights-off except for the short photoperiod with 2 h of twilight where activity onset was on average 5.3 (SEM 1.07) h after lights-off. This finding contrasts with the results of Boulos and Macchi for Syrian hamsters where a 5.9-h earlier activity onset was observed when similar photoperiod and twilight conditions are compared with a rectangular LD cycle. The authors suggest the opposite effects of 2 h of twilight in the 2 species may be related to their different free-running periods under DD conditions following entrainment to short photoperiod with 2-h twilight conditions. Since the authors observed larger variation in phase angle of entrainment in longer twilight conditions, twilight does not necessarily form a stronger zeitgeber.
authors.affiliation: Department of Chronobiology, Center for Life Sciences, University of Groningen, Haren, The Netherlands.
authors.forename: M
authors.initials: M
authors.lastname: Comas
authors.forename: R A
authors.initials: RA
authors.lastname: Hut
mesh_terms: D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D001683:Biological Clocks / Q000502:physiology*; D002940:Circadian Rhythm / Q000502:physiology*; D003624:Darkness; D008027:Light; D051379:Mice; D008810:Mice, Inbred C57BL; D009043:Motor Activity / Q000502:physiology*; D010507:Periodicity; D017440:Photoperiod