Hibernation is a time each year in which some animals go into a deep sleep. This happens to some mammals that live in cold places, such as bears, skunks, hamsters and some bats. While sleeping, the animals breathe more slowly, slow their heartbeat, and lower their body temperature. Before going into hibernation, each animal stores fat by eating a lot more than usual. That way, it can live off the fat during the long, cold winter months when food is not easily found. Bears make their dens in a dark, hidden, dry, sheltered place. They may snooze away in a cave, under large roots, in a hollow or under a fallen tree. Bears may also dig a den in the ground or in the ice. Once they find a good spot, they arrange leaves, grass, tree bark, moss and other bits that fall to the forest floor into a bed. Most of these materials will last through the winter without falling apart.