It's hard sometimes getting a good night's sleep. But it's dangerous not to. In all sorts of ways:

Sometimes, a lack of Zzzs can be attributed to not enough time to sleep, or to too much stress, or to snoring or sleep apnea. But it goes deeper than that for many.

An estimated 50 to 70 million adults in America have a straight-up "sleep or wakefulness" disorder, according to the CDC. The Stanford Center for Sleep Sciences and Medicine identifies more than 100 such disorders, and some are downright scary. Here are five of them.

1. Night terrors

A frightened girl in bed pulls the sheets up her facePeople typically experience night terrors — thrashing, screaming — early in the sleep cycle. (Photo: CREATISTA/Shutterstock)

Also called "sleep terrors," these are included in a group of disorders known as parasomnias. According to the researchers at Stanford, they involve "abnormal behaviors," and can occur at any age.

Night terrors are just that: Episodes of extreme terror while asleep. People who suffer from night terrors thrash about, moan, scream — all while not fully conscious. You can try to soothe someone with night terrors, but you usually won't get a response. Often a person experiencing night terrors drifts back into regular sleep and doesn't remember anything upon waking. According to the Stanford researchers, these episodes usually occur early in the night.

2. Exploding head syndrome

A man holds his head while lying in bedA loud noise can disrupt your attempt to sleep, and it ends up feeling like your head is about to burst apart. (Photo: Eugene Sergeev/Shutterstock)

Say you're just starting to drift off, and somebody creeps up to your ear and crashes a pair of cymbals together. Or you're about to settle into a sleep and a loud noise — it sounds like a firecracker right next to your skull — goes off. That's where exploding head syndrome gets its name.

Researchers at Washington State recently detailed a surprisingly high percentage of young people — around 18 percent — who have been affected by this psychological phenomenon. It can significantly alter sleep and negatively impact lives, the study found.

Exploding head syndrome can lead to another sleep disorder, too, known as …

3. Isolated sleep paralysis

In deepest sleep, the body is at rest, largely unresponsive to the brain. A sudden interruption of that sleep can lead to the mind waking but the body remaining at rest — a temporary paralysis, lasting from several seconds to several minutes. When you're awake and you can't move, a good deal of panic also can set in.

Regular sleep paralysis (not the isolated kind) is a type of narcolepsy, often characterized by excessive sleepiness during the day that is not caused by problems sleeping at night. People with narcolepsy often fall quickly, deeply into sleep. Many experience cataplexy, "suddenly feeling paralyzed or weak in the head, legs or other body parts especially after excitement or laughing," according to Stanford.

4. Kleine-Levin Syndrome (KLS)

A blonde woman sleeps soundly in bedPeople with Kleine-Levin Syndrome will sleep for long stretches of time, hence the condition's other name: Sleeping Beauty Syndrome. (Photo: Elena Elisseeva/Shutterstock)

This condition, colloquially known as Sleeping Beauty Syndrome, is characterized by bouts of excessive sleeping — most of the day and night — and can last from a few days to several weeks. When people with KLS are awake during a bout, they're often confused, super-hungry and can have uninhibited hypersexuality.

In between bouts, though, people with KLS are asymptomatic, sleeping and waking normally. The condition most often strikes teens.

5. Circadian rhythm disorders

Sleepless man holds TV remote His body is ready to sleep, but his brain is really into those late-night reruns. (Photo: danilobiancalana/Shutterstock)

We've all had a little bit of this at one time or another: your "internal clock" is thrown off and your body says, "Sleep!" but your mind says, "What's on TV?"

This can be caused by something as simple as going to bed a little too late, which makes you late getting up; working that overnight shift, which has you wide awake at 3 a.m. even when you're not working the overnight any more; or simple jet leg issues, where your body says it's 1 a.m. back home in New York, even though things are just getting started at 10 p.m. while you're in L.A.

Another odd circadian rhythm disorder: Non-24, a rare condition in which a person can't adjust to a 24-hour sleep/wake cycle. Many people with Non-24 sleep well; they just have trouble getting there or getting up once they're finally asleep. More than half the blind population suffers from Non-24.

The National Health Interview Survey says that nearly 30 percent of adults report an average of six hours or less of sleep per day. Considering adults need at least 7 to 8 hours a night, a lot of sleepy people are stumbling around.

The importance of sleep shouldn’t be taken lying down. "If your job or daily routine limits your ability to get enough sleep or sleep at the right times, talk with your doctor," the NHLBI says. "You also should talk with your doctor if you sleep more than 8 hours a night, but don't feel well rested. You may have a sleep disorder or other health problem."

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