Technically, snakes do not hibernate; they brumate. When cold weather sets in, they burrow underground or find other protected hiding spots. Once snakes are settled in, they rest. A snake stays in this state until environmental changes in temperature, pressure and amount of daylight signal a return to warmer weather.
However, if there is a warm spell during cold weather, snakes sometimes resurface for that interlude. They return to their dens when the weather changes again.
During cold periods, snakes must brumate because they are not able to keep their body temperature warm enough to survive. To maintain brumation, snakes do not eat, so their digestive system slows. Their heart and respiratory rates reduce as well.