Horror movies affect everyone differently. Some people can sit through an entire marathon and it’s no big deal, while others will be left with some very real feelings of fear and distress. Even knowing that the images seen on screen aren’t real, why do scary movies give some people nightmares? 

Nightmares are terrifying dreams that can wake you up and keep you awake for the rest of the night. They usually occur in the later hours of rapid eye movement sleep. Rapid eye movement sleep (REM sleep) is a unique phase of sleep characterized by random and rapid movement of the eyes, low muscle tone throughout the body, and the presence of vivid dreams. People enter REM sleep within the first 90 minutes of falling asleep and, as the sleep cycle repeats throughout the night, REM sleep occurs several times nightly. During REM sleep, the brain is almost as active as it is when we are awake. 

If watching a horror movie makes you feel scared and anxious, it may play out in your dreams as well. “We’re wired to develop some anxiety in response to a threat,” Sunny Volano, a Licensed Professional Counselor and therapist specializing in anxiety, tells Bustle. “While movies are not a real threat, they create the same fear in your brain, and it has trouble telling what’s real versus imaginary. Scary movies can cause someone to have nightmares because it triggers the fight or flight response.”

According to Volano, it’s why people with anxiety or trauma are more prone to getting nightmares. If someone is always slightly on alert, they may be more sensitive to scary images like blood, gore, and violence. “These images can get stuck in your head and can lead someone with anxiety to feel fearful over what could happen to them or someone they love,” Volano says.

“Dreams are stories created by our minds to help us express and expel complicated emotions,” Pam Muller, dream expert and author, tells Bustle. “In fact, noted dream researcher Ernest Hartmann, M.D. said that dreams contextualize our emotions. It’s our biological attempt at learning from our heightened emotional experiences.”

When you have a “sudden spike” in emotional response, like getting scared from a terrifying image on screen, your brain takes note of that experience and sees it as a learning opportunity. After a scary movie, your brain tells itself that there’s something here worth paying attention to.

“Nightmares after horror films are our mind’s attempt to feel the full extent of the emotions that arose during the screening, specifically emotions that we didn’t allow ourselves to fully realize or feel at the time,” Muller says. It’s your brain’s way of working out the scary experience so you can avoid having another negative response like that again in the future.