Claiming the movie you watched last night was “bloodcurdling” may not be a hyperbole after all — a new study out in The BMJ Christmas issue has suggested watching horror movies is associated with an increase in the clotting protein, blood coagulant factor VIII.

Apparently, the folks who coined the term “bloodcurdling” to describe extreme fear were on to something. Back in medieval times, the term evolved from the idea that fear could “curdle” the blood or “run the blood cold,” but its validity had never been studied until now.

Researchers from the Netherlands decided to assess whether acute fear really does clot up the blood, which they say poses an important evolutionary benefit — the body preparing for blood loss in life-threatening situations. The team gathered 24 healthy volunteers to watch a couple films and have their blood drawn.

Fourteen participants were assigned to watch a frightening horror movie, followed by an educational, non-threatening movie. The other 10 were to watch the movies in reverse order. Everyone watched their assigned movies more than a week apart, and at the same time of day, getting blood drawn before and after each movie.

The team analyzed the blood samples for markers or “fear factors” of clotting activity. Participants were also responsible for rating their fear on a visual analogue fear scale ranging from 0 (no fear) to 10 (the worst fear imaginable). Predictably, the horror movie was rated as more frightening than the educational movie, with a 5.4 mean difference in fear rating scores. The difference in coagulant factor VIII, the clotting marker, levels before and after watching the horror movie was higher than for the educational movie.

Levels increased in 57 percent of the participants during the horror movie, but only 14 percent during the educational movie. Levels of coagulant also decreased in 86 percent of the participants during the educational movie, but only 43 percent during the horror movie. Interestingly, the team found no effect on other clot forming proteins from either movie, suggesting that coagulation may be triggered by acute fear, but this does not lead to actual clot formation.

The researchers point out some study limitations, but ultimately conclude that “in young and healthy adults, watching bloodcurdling movies is associated with an increase in blood coagulant factor VIII without actual thrombin formation.”

Source: Nemeth B, Scheres L, Lijfering W, Rosendaal F. Bloodcurdling movies and measures of coagulation: Fear Factor crossover trial. The BMJ. 2015.