Why Do We Get Nightmares? There’s A Biological Purpose Behind Your Bad Dreams

If you've ever woken up with your heart pounding and your teeth on edge after a particularly frightening or negative dream, you're far from alone. Nightmares are most common among children, according to the National Sleep Foundation, but adults experience them regularly; up to 5 percent of the adult population has a "nightmare disorder," according to the Mayo Clinic, where nightmares are so common and distressing that they start to affect everyday functioning. But virtually everybody has experienced at least one upsetting or miserable dream, particularly if they've done something to trigger it — like watching a scary movie late at night. But why do we have them? It turns out that nightmares are actually an interesting aspect of brain function, and don't just function as a way to make us sweat in our sleep.

A 2014 survey of the content of nightmares found that people didn't necessarily experience terror in their nightmares; they felt a wide range of emotions, like disgust, shame, confusion, guilt, and sadness. And at the worse end of the spectrum, there are "night terrors", which are distinguished from nightmares by their severity and physical reactions. People with night terrors, both children and adults, may scream and thrash while they're asleep, but they likely don't remember the terror the next day. Nightmares, by contrast, are confined to the dream world — but that doesn't make them any less horrible. Fortunately, ish, there are some reasons they happen that actually make some sense.

1They've Evolved To Keep Us Safe

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Nightmares have stuck around for a reason. Whenever you feel terrified by a specter in a dream, it's likely because your neurology has evolved to keep you on your toes about potential threats. Harvard psychologist Deirdre Barrett told Live Science that nightmares "may have been useful in ancestral times when a wild animal that had attacked you, or a rival tribe that had invaded might well be likely to come back." If you keep reliving an upsetting experience in your nightmares, it's likely because your brain is attempting to remind you to avoid it and keep yourself safe.

However, not all nightmares are reactive. While for generations it was thought that all dreams were reflective of life experience, it's since been understood that they're very complex things, and that includes nightmares too. So what good does a nightmare about being chased through a haunted house by a giant Hello Kitty — which is likely never really going to happen — do us in the real world?

2They Might Be "Dress Rehearsals"

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There's a theory that nightmares aren't just reactions; they're also preparing us for the future. In other words, they don't just replay things that we're afraid of. They also set us up to deal with new fears that may come. Professor Allan Hobson, Professor of Psychiatry, Emeritus at Harvard University, wrote in the New York Times that he has a theory about nightmares: that they're effectively dress rehearsals for experiencing bad feelings, including terror and anxiety, in our waking lives. "The individual who can have bad dreams," he wrote, "has a better chance of survival than one who can't."

Part of his evidence for this is the fact that nightmares only occur in deep REM (rapid eye movement) sleep, a part of rest which only occurs in mammals and birds, which have been evolving continually for millions of years. Nightmares, he notes, seem to spring from our "innately fearful and innately angry" brain, and help us survive in a world that can be threatening and infuriating.

However, a study reported in Scientific American in 2010 found that people who'd had nightmares were more vulnerable when they saw terrifying images the day after than people who'd had normal dreams. In other words, nightmares made people more fearful, not less — at least in the short term.

3Our Amygdalas Are Over-Active

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The amygdala, which is responsible for detecting and responding to threats, is one of the most active parts of the brain when it's having a nightmare. It's wide awake when you're having REM sleep, and the more activity it shows, the more likely it is to be giving you bad dreams; studies have linked an over-excited amygdala to nightmares, particularly in people with PTSD. Amygdala activity in the sleeping brain has been linked to emotional "processing," as the brain attempts to digest emotional signals and sort out confusion, but when it goes into overdrive, it can stop processing signals properly and start showing you a reel of very negative emotions.

4We've Experienced A "Nightmare Trigger"

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A lot of things can set nightmares off, and they're very different for everybody. Eating late at night can provoke a more active amygdala because it induces the brain to be more "awake," so that midnight snack could genuinely be causing issues. Anecdotal evidence links everything from spicy food to vitamin B6 to a higher nightmare occurrence, though it's likely that these are highly individual triggers and won't necessarily be the case for everybody.

However, there's more solid information about certain medications: it's thought that antidepressants don't actually cause more nightmares, but people who take them are more likely to remember the dream in the morning, and so believe they're experiencing them more. Sedatives, beta blockers and amphetamines are also linked to a higher nightmare frequency, though it's not known why.

The upshot is that, if we understand why we're experiencing nightmares, we can work to mitigate those causes or triggers; if you know that your night cheese leads to a dream where you're giving a presentation in your underwear, you might consider curtailing your cheese habit to an afternoon snack. Similarly, understanding the biological reasoning behind a nightmare may make it easier to handle, even if it doesn't make it any less scary. Sleep tight, friends.