Equation representing the dark-entrained transition from inaction to action in male and female mice.

•Discovery that the transition from low to high activity/arousal can be described by a relatively simple equation.•And the reverse, the transition from high to low follows the same equation, time-reversed.•The equation applies, even to arousal/activity in constant darkness.•The behavioral transition is sensitive to sex hormones.•The activity/arousal schedule followed by the US Navy leads to abnormal behavioral rhythms.