Bears spend months in winter dens in a state that is lighter than true hibernation.

The longer version

A denning bear lowers its metabolism and lives off stored fat for months without eating, drinking or eliminating waste, yet body temperature falls only modestly and the animal can be roused fairly easily. The state sits between ordinary sleep and full hibernation. Muscles waste surprisingly little, and bone density is largely preserved despite months of immobility.

True hibernators such as ground squirrels drop body temperature close to ambient and must rewarm periodically. Bears avoid the cost of that rewarming. Cubs are born during denning, and the mother nurses them while still in this reduced state. Biologists have studied the physiology closely, partly because the same mechanisms interest researchers working on muscle wasting and bone loss.

Where this entry sits

This collection gathers curious details about sleeping and dreaming, from nightly brain stages and body clocks to naps, yawns and unusual animal habits. It also covers how bedtime customs have changed across centuries, without offering any advice. The full list sits on the Sleep and Dreams page, where every entry is listed in order.