Bears don't hibernate. They sleep a lot in bad weather but they aren't true hibernators like groundhogs. Hibernation is a physiological phenomenon that's characterized by very specific criteria such as extremely low body temperature and basal metabolism. A groundhog in hibernation is only just barely alive. A bear in his winter sleep is close to normal body temperature, or at it; a groundhog is easily 25 degrees (Celsius) below the normal 37 degrees. If you poke a sleeping bear with a stick he will wake up, though he may not do much. If you poke a sleeping groundhog he is totally unaware of it. The only way to wake him is to put him in a warm space and let him come to that way.
There are many other features of hibernation state, too many to go into here, but among them are changes in blood flow, blood viscosity, and neural wave patterns, that non-hibernators don't display. Hibernation is so profoundly different a state of being than normal life that very few animals have developed it. It's unlikely that an animal the size of a bear could do it: there's an upper body size limit that's just about the size groundhogs reach.
The normal awakening mechanism is really interesting: there's special tissue in the animal that is specifically a heat-generation site. the blood carries the heat to the rest of the body and awakening is a slow process. The whole thing is controlled by hormonal signals that relate to day length, temperature, body weight, fat content, etc. Many a PhD dissertation has been written on hibernation, it is a fantastically complex phenomenon.
Animals that go into daily torpor have similar mechanisms but they are nowhere near as extensive as in a hibernator in full depth. Bats go into daily torpor, where they let they body temperature drop a bit; then they warm themselves up at night. But it isn't really the same thing, it's a short-term version of it.