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What Causes Nightmares?

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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.

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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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Big Questions
How Are Speed Limits Set?
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When driving down a road where speed limits are oppressively low, or high enough to let drivers get away with reckless behavior, it's easy to blame the government for getting it wrong. But you and your fellow drivers play a bigger a role in determining speed limits than you might think.

Before cities can come up with speed limit figures, they first need to look at how fast motorists drive down certain roads when there are no limitations. According to The Sacramento Bee, officials conduct speed surveys on two types of roads: arterial roads (typically four-lane highways) and collector streets (two-lane roads connecting residential areas to arterials). Once the data has been collected, they toss out the fastest 15 percent of drivers. The thinking is that this group is probably going faster than what's safe and isn't representative of the average driver. The sweet spot, according to the state, is the 85th percentile: Drivers in this group are thought to occupy the Goldilocks zone of safety and efficiency.

Officials use whatever speed falls in the 85th percentile to set limits for that street, but they do have some wiggle room. If the average speed is 33 mph, for example, they’d normally round up to 35 or down to 30 to reach the nearest 5-mph increment. Whether they decide to make the number higher or lower depends on other information they know about that area. If there’s a risky turn, they might decide to round down and keep drivers on the slow side.

A road’s crash rate also comes into play: If the number of collisions per million miles traveled for that stretch of road is higher than average, officials might lower the speed limit regardless of the 85th percentile rule. Roads that have a history of accidents might also warrant a special signal or sign to reinforce the new speed limit.

For other types of roads, setting speed limits is more of a cut-and-dry process. Streets that run through school zones, business districts, and residential areas are all assigned standard speed limits that are much lower than what drivers might hit if given free rein.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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