The pumpkins are out; the plastic, fake-blood-covered knives are in hand, and whatever Paranormal Activity sequel we're up to is in theaters. It's definitely the spookiest time of the year, but what's actually happening when we get spooked? Well, this new Reactions video from the American Chemical Society is here to uncover the mystery of terror — whether it's all in our heads or we're actually in very real danger of dying.To kick off a scary experience, you might hear a horrifying scream. And research suggests that our brains process screams differently from normal speech. In particular, they're funneled through the amygdala, which acts like an alarm system and sets off a series of freaked-out reactions.That fear is created by chemical messengers in your brain alerting you to the fact that danger is most definitely afoot. These messengers include glutamate, which eventually sends a signal to your brain's hypothalamus, a major player in your fight-or-flight response. It ramps up your adrenaline levels and spurs your liver to release energizing glucose, like a little sugar rush. And, of course, our old friend cortisol (a.k.a. "the stress hormone") keeps these processes going at full blast.Exhausted? So are we. At this point, with any luck, you've gotten away from whatever threat was after you, giving you a chance to rest. If not, we're so sorry. At least you'll finally get the chance to catch up on a lifetime's worth of sleep