Beeing outside in the dark. Not in the streetlamp dark but the forest dark. If you can't see anything then nothing can see you. It's like a blanket that covers everyones eyes.
Predators can see in the dark much better than you do and not only that; their sense of hearing and especially smell is orders of magnitude better than yours.
As someone who has spent several nights alone in a pitch dark forest, shit's scary. You hear sounds around you all the time and you have no idea wether it's a bird, human or a bear.