According to my experience and research (others knowledge, my interpretation) Only bears at upper elevations and climates "hibernate" those in less extreme altitudes and climates can enter a period of "Torpor" , not true hibernation.
Evolution implanted the need mostly due to lack of food availability during winter, but in warmer climates it is not a necessity.
The only real need is due to reproduction, although mating season is in the spring, the embryo does not turn into a zygote and implant itself into the uterine wall until the bear (sow) enters this state in the fall, and even then only if the sow has enough fat reserves to support a fetus (or 2-4)
They can, and will wake up for a variety of reasons because it is not a true hibernation, there was even a study done on Long Island due to a drop in cubs, found out a boar was going around and killing cubs while still in den.
But if the sows are in their dens with young, the boars have no reason to wake up from their naps, the only reason they get moving in the spring is when they get hungry, that has to do with new growth of plants, bringing us back to photoperiod and weather...
It will be around the same time every year due to photoperiod, but amount of traveling, and feeding will be directly related to weather.
Again, put a bird feeder up, when the squirrels start showing up, the bears are moving...
