How are the sled dogs able to sleep without freezing?

They bury themselves in the snow. The snowpack combined with their coat provides an insulating cover that keeps them alive and dry.

Sled dogs have frozen to death in the Iditarod in blistering cold winds. The dogs also get frostbite (where it hurts the most!), on their flanks, ears and tail. The frostbite is very painful.