Women's periods attract bears. Bears can smell the menstruation. Therefore, when a woman is on her period, it is called "Bear Season".
Dude, did you bang her last night?
Nah, its bear season.
The urge to go to bed at 7PM when it gets dark very early in the northern hemisphere.
"Wanna go get a beer?"
"Can't: Bear Season."
"I fell asleep at six and was wide awake by 3 AM"
"yeah man, that's Bear Season for you"