There’s a reason people are afraid of things that go bump in the night. Once the sun goes down, people are more sensitive to scary stimuli. That jumpiness may have more to do with the time of day than the darkness, a new study in the International Journal of Psychophysiology explains. 

A group of Chinese researchers tested the links between fear, darkness, and nighttime by splitting a group of women into four groups and exposing them to scary sights and sounds; the women were monitored for spikes in heart rate and perspiration. Some participants looked at scary pictures and listened to scary sounds during the day with all the lights on, while others looked at the pictures during the day, but in darkness. Some viewed them at night with a dim light, and others at night in complete darkness except for the computer screen. 

Those who undertook the task at night found the pictures and sounds scarier than did the women who had to examine them during the day, regardless of the light condition. By contrast, their reaction to neutral pictures and sounds didn’t vary based on the time of day. 

The study only tested young female participants in China (the average age was 22), so it's far from definitive. Fear could affect men and women and people of different ages and cultures differently. But this does suggest that there could be a circadian rhythm to fear, and that biologically, we’re more inclined to be afraid of things at nighttime. As humans aren’t nocturnal animals, but plenty of predators are, it makes sense that we would learn to be a little more sensitive to threatening sights and sounds in the night. 

[h/t BPS Research Digest]