It turns out, we can thank our caveman ancestors! According to sociologist Margee Kerr, our ancestors knew, once it was dark, that’s when predators or rivals might attack. And even after all this time, our brains still have so-called “evolutionary anxiety” associated with darkness. So, we may not be afraid of a saber tooth tiger anymore. But once night falls – somewhere in the back of our mind – we feel vulnerable and stay on alert.