Twilight and Photoperiod Affect Behavioral Entrainment in the House Mouse (Mus musculus)

The onset of activity in C57BL/6JOlaHsd mice occurred on average 5.3 hours after lights-off during a short photoperiod with 2 hours of twilight.

• For all experimental conditions, activity onset was close to dusk or lights-off.
• A significant difference in activity onset was noted under the 2-hour twilight condition compared to other conditions.
• The observed activity onset contrasts with findings in Syrian hamsters, which showed an earlier activity onset under similar conditions.
• The variation in phase angle of entrainment increased with longer twilight durations.
• Twilight does not necessarily act as a stronger time cue (zeitgeber) for these mice.

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