Does being in the dark ever send a shiver up your spine? Have you ever hurried down a hallway after turning out the light? You may get spooked by suspicious shadows in a dark room only to turn on the light and realize it is just an innocent chair. Then, you might wonder if there are really ghouls lurking in the dark or if it’s simply a trick of your brain. The answer to these questions lies in a pair of small almond-shaped structures in the center of the brain called the amygdala.

Horror movies and haunted houses rely on the unease and fear people feel in the dark to elicit a larger fear response for a bigger thrill. It turns out that it’s more the absence of light than the actual darkness that makes your heart pound and causes you to feel uneasy. Natural light plays a significant role in human health: It regulates circadian rhythms and improves mood, alertness, cognitive function and focus. This is where the amygdala—the brain’s “fear center”—comes into play.

The amygdala lights up with activity when it perceives a threat and initiates the “fight or flight” response, which makes your heart pump more strongly and your breath quicken. When light enters the eye, information is sent throughout the brain. The vision center interprets the image, your body clock synchronizes the body’s rhythms and activity in the amygdala is suppressed so that the fear response isn’t activated.

If you think of the amygdala as the gas in a car, then another part of your brain, called the prefrontal cortex, is the brakes. When an object startles you in the dark, your prefrontal cortex recognizes that it is simply a chair and not a threat. This part of your brain stops the fear response before it gets out of hand. In the light, these two regions are perfectly balanced in a healthy brain. However, in the dark, the system breaks down. In the absence of light, the amygdala becomes more active and the prefrontal cortex becomes less active, making the shadows you perceive in the dark seem more threatening.

Scientists think that evolution is the reason that anxiety builds in the dark. Sleeping at night makes you more vulnerable and more cautious, and therefore, in primitive times, more likely to survive. So, the next time a strange shadow at night gives you a fright, remember your brain just wants you to turn on the light!

Gillian Kelly is a recent graduate of the University of California, Berkeley. She is a biomedical technician at Tripler Army Medical Center in Honolulu. Kelly plans to pursue a PhD in neuroscience. The views expressed in this post are those of the author and do not reflect the official policy or position of the U.S Department of the Army, U.S. Department of Defense or the U.S. government.