They come from many transmigrations in species which depend totally on sun energy and cannot be sustained otherwise. An example is alligators who need to be in the sun for some hours sometimes. The reptile case is not extreme but it gives some practical way to regard this. There is the story about Kumblakarana, who used to sleep for months at a time and could not get up even if elephants trampled his body. That is in the Ramayana of Valmiki. Ravana, the villian was the brother of Kumblakarana.