Do bears really sleep through an entire winter?

No. Denning bears, such as brown and black bears, often retire to their caves for the winter months, but they sleep lightly, and are often active, with females giving birth to cubs during the winter. They don’t technically hibernate during these months. Hibernation is when a species passes the winter in an inactive state, conserving their resources and energy until winter passes.

Bears are not true hibernators because their body temperature drops only a few degrees and they show only a moderate drop in their metabolism. Small animals with high metabolic rates such as rodents, hummingbirds, and bats are true hibernators: their body temperature drops almost to the level of the surroundings and they show little response to nature’s sights and sounds. These animals collect and eat a lot of high-calorie foods (such as nuts) to store calories to make it through the hibernation period.