What Causes Nightmares?

When you're waking up from a nightmare, your first question might be, “Was that a bagel chasing me through my house with a sledgehammer?” And after the shock of dreaming about a homicidal, anthropomorphic breakfast dish wears off, your next question is probably, “Why was that bagel chasing me through my house with a sledgehammer?"

Nightmares, and dreams in general, occur during the rapid eye movement (REM) sleep stage. Depending on how long you sleep, your body goes through four to six cycles a night, and the REM stage gets longer with each sleep cycle. Most nightmares happen during the last third of your night’s sleep.

For most people, nightmares aren't a major problem: Only five percent of adults have a clinical nightmare problem where the dreams are too severe or frequent. But 85 percent of adults still experience normal nightmares—8 to 29 percent of people claim to have nightmares on a once-a-month basis, and two to six percent have nightmares once per week. 

Experts say anything from everyday stress to trauma (nightmares are common in post-traumatic stress disorder) to just good-old-fashioned watching scary movies might trigger nightmares. But if you want to dodge a restless night pockmarked by bad dreams, you might want to rethink having that pre-bedtime candy bar.

Was it something I ate?

Eating anything right before bed boosts metabolism and temperature, according to the Cleveland Clinic’s Sleep Disorder Center. That upsurge leads to more brain activity in REM sleep, prompting more dreams. One Canadian sleep study showed that, of 389 subjects, 8.5 percent blamed bouts of bad dreams on food.

Biochemists at Australia’s University of Tasmania conducted a study where they added mustard and Tabasco sauce to the dinner plates of six “young, healthy male subjects.” The spicy kick of the condiments “elevated body temperature during the first sleep cycle” and increased the subjects’ total awake time and sleep onset latency, or the time it takes to go from fully awake to fast asleep.

It’s not just the spicy stuff to watch out for, though. An article published by The Journal of the Mind and Body recapped a study that showed that junk food—ice cream and candy bars were used in the experiment—triggered more brain waves, causing seven of ten participants to experience nightmares.

How you sleep also plays a role in what kind of dreams you’re in for. A 2004 study found that left-side sleepers experience significantly more nightmares than right-side sleepers. And according to Prevention magazine, sleeping on your stomach—the least popular sleeping position—leads to the most emotionally charged dreams.