According to the article, one idea is it's because they slept on the ground and were more vulnerable to predators. But then speculates that it could also be bec

According to the article, one idea is it's because they slept on the ground and were more vulnerable to predators. But then speculates that it could also be because people were better at using dark time more productively than other animals, thanks to story telling and fires. If I can add my own speculation, I think we are our own predator and selection pressure. People haven't been threatened by nature for a very long time, but we are threatened by each other.