Everyone will have bad dreams at some point (or many points) in their lives. They can range from uncomfortable to horrifying to downright dangerous. Some bad dreams are severe enough to be considered parasomnias, or sleep disorders. Here's a glimpse at what some of the most unsettling parasomnias have in store for the people who experience them.

1. NIGHTMARES ARE NOT MERELY BAD DREAMS.

A nightmare is indeed a bad dream, but not all bad dreams are nightmares. Specifically, a nightmare is a dream with a vivid and disturbing plot that most often wakes the dreamer from their sleep. Upon waking from a nightmare, a person may be sweaty, out of breath, and feel afraid. A bad dream, on the other hand, is simply a non-medical catchall term for dreams that are unpleasant.

Nightmares take place during the final and deepest phase of sleep (rapid eye movement, or REM), when the brain shuts off communication with the spinal cortex, causing our limbs to be temporarily paralyzed for sleep. Research has linked PTSD, anxiety, and certain medications to nightmares in adults. Nightmares can also be caused by eating before bed. Eating triggers a metabolic response and increases the brain’s activity, even during sleep.

2. ADULT NIGHTMARES ARE SOMEWHAT RARE.

Frequent nightmares among children ages 5 to 12 are pretty common, affecting 20 to 30 percent of kids, according to the American Academy of Family Physicians. Nightmares most often taper off as children reach adolescence though, so in adults, frequent nightmares are somewhat rare: Only about 5 to 8 percent of adults will experience frequent nightmares, and generally only certain personality types will have this experience.

Overall, adults who are most likely to suffer from frequent nightmares are those who are "creative" (those who on psychological tests, as this sleep study says, are more likely to reject rigid understandings of the world and interpret life in shades of gray), or might have an underlying psychological problem. A 2015 study of Finnish adults found that adults who deal with depression, exhaustion, and insomnia are much more likely to have nightmares. Another study from 2009 demonstrated a link between anxiety and increased nightmares for women. This same report found that adult women are more likely to experience nightmares than men.

3. SLEEP PARALYSIS CAN BE SCARY AS HELL. LITERALLY.

Anyone who has had sleep paralysis knows how terrifying an experience it can be. The sleeping person’s mind is essentially awake, but the body is immobile and in a state of sleep (hence “paralysis”). Sleep paralysis only occurs in stages of light sleep and is most likely to happen during a nap or close to the end of a night’s sleep. A person will experience visual and auditory hallucinations that are universally fearsome in content. The body is unable to move during an episode of sleep paralysis, making the experience all the more surreal and terrifying. Neurobiologists have found evidence suggesting that the paralysis experienced during sleep paralysis could be caused when shifts between REM stages don’t happen as they should, resulting in a brain that is awake and a body that remains asleep.

A very common sleep paralysis hallucination is of a demon sitting on the chest of the sleeper; it's often accompanied by the sound of whispers, train horns, or bells. Across cultures, hallucinations that result from sleep paralysis involve an intruder that is perceived as a threat, whether it's the succubus of medieval Europe, the Old Hag of Newfoundland, the alien abductor of America, or the Batitat of the Philippines.

4. NIGHT TERRORS CAN MANIFEST IN UNSAFE WAYS …

Night terrors are a whole other level of body and brain activity that are highly disturbing—but unlike nightmares, night terrors don't awaken the sufferer, despite manifesting as an extreme experience. During a night terror, a person might thrash, scream, or yell, have open eyes, and be extremely difficult to awaken. It’s also not uncommon for someone in the throes of night terrors to physically act out, which could be very dangerous if the person were to leave bed or the home while still asleep. Night terrors are not dreams, per se, but the result of a malfunction in the brain that occurs when the sleep stage moves from light to REM sleep.

5. … BUT REM BEHAVIOR DISORDER IS EVEN MORE DANGEROUS.

One of the most unsettling forms that dreaming can take is REM behavior disorder (RBD). Brought into the mainstream by comedian Mike Birbiglia’s 2012 film, Sleepwalk With Me, RBD is present when sleeping people act out the narrative of their dreams, which is likely to include risky behavior (such as jumping out of a closed second-floor window, as Birbiglia did). The experience of RBD is similar in some ways to night terrors, but the content of RBD dreams will be generally involve a lot of action—running, jumping, playing sports—and its manifestations can be violent.

As the name suggests, RBD occurs during REM sleep and is commonly associated with disorders such as sleep apnea, narcolepsy, and periodic limb movement disorder. When a person suffers from RBD, typical muscle atonia does not occur during deep sleep, which results in physical action. It is not uncommon for sufferers to hurt themselves—or anyone they share a bed with.

Any of this ring a bell? Check in with your doctor. The occasional nightmare isn’t cause for concern, but a doctor will want to know about any sleep disorder behavior that persists—or causes injury. But if you’re in the majority of fortunate adults who have pleasant dreams without incident each night, continue to sleep tight!