Did you know that many small animals that stay in cold environments during the winter go into a brief daily hibernation-like state? Scientists call it torpor. Torpor allows the animals to lower their body temperatures greatly, and their breathing and heart rate are so slow you can barely tell they're alive.
Since these animals are so small, they can't store loads of energy in fat like bears can, they have to go out every day and eat as much food as they can. However, lowering their body temperature and almost shutting down their body while they sleep means they can also save a lot of energy during the night.
Only small animals show daily torpor. Big animals can't do that because it takes time to get the larger body going and back up to a normal temperature. The day would already be over and it would be getting cold again before they were even warm enough to move. This is a special adaptation that small animals like mice, hamsters and hummingbirds can use to help them survive in environments where it gets really cold at night.