Groundhogs are typically active until the first real frost of winter, often October or November. Then they crawl down into their burrows and begin to hibernate. This hibernation lasts until the weather starts to warm up, usually around February or March. Although they can hibernate for up to five months, some hibernate for only two to three months, depending on temperatures in the area. As the days shorten, temperatures drop and plants begin to die or go dormant for the winter, a groundhog's body releases a hormone that helps guide the animal into hibernation until the weather warms and the days begin to grow longer.