Sometimes there are aspects of the brain designed for one purpose that get used for a total different one. For example, neurons that detect smell will trigger w

Sometimes there are aspects of the brain designed for one purpose that get used for a total different one. For example, neurons that detect smell will trigger when we are thinking about purely moral issues, possible because it helps to boost the emotional reaction. The body do cool down when it sleeps, and many people do feel aided in falling asleep if they lower their body temperature a bit. Yawning might just be that, a small trick to get the body to do a short temporary jump towards a more sleepy state, and a social ques with a minor biological aspect to trigger similar state in a social group. It could also be the other way around, that being a social trigger with a minor biological aspect attached that happens to have a real use in other animals.