Turns out, sleep isn't defined by the binary way we describe it: You aren't simply awake or asleep. Instead, there are other states of slumber -- simple disturbances like dreams, and more troubling conditions, like night terrors and sleep walking.
There are also curious medical conditions like narcolepsy and restless legs syndrome.
And then there are even stranger scenarios that we hear about much less frequently. Thanks to their rarity, some are still mysterious even to the people who study sleep. Here are some that stump the experts:
Sleep Sex
So-called sexsomnia is allegedly like any other behavior acted out while sleeping, like sleep walking or eating or talking. These behaviors occur when part of the brain is awake but the rest of it is asleep. Sexual behavior during sleep can be minimal or as serious as intercourse, oftentimes rough, which in some cases has led to legal proceedings over unwanted contact. In fact, experts have cleared accusations of assault and rape because a defendant was technically sleeping at the time. "It is instinctive behavior, people are not conscious at the time," Chris Idzikowski, Ph.D., director of the Edinburgh Sleep Clinic told the BBC. "When you are in a deep sleep, moral and rational decision-making do not occur."
Sleep sex seems to occur in people with what is called parasomnia overlap disorder, meaning something is off with both REM and non-REM sleep, according to a 2011 study. There is currently no approved treatment, but some sedatives and antidepressants have shown some off-label success, according to Everyday Health. Experts recommend keeping the person with sexsomnia safe with measures such as "sleeping in a separate bedroom, locking doors or even putting alarms on doors to wake people up and make them aware of what is happening," Raman Malhotra, M.D., co-director of the Saint Louis University Sleep Disorders Center, told the website.
Kleine-Levin Syndrome
Sometimes referred to as Sleeping Beauty syndrome, KLS is a very rare disorder that causes people to sleep up to 20 hours a day, according to the National Institutes of Health. About 70 percent of people with KLS are male, and most are teens. Typically, extended sleep will happen over a number of days or weeks, and the intensity and frequency of these so-called episodes generally decrease over time. During an episode, people with KLS are often too tired to leave bed even when awake, according to the Kleine Levin Syndrome Foundation.
KLS has been linked to a problem in the parts of the brain that deal with appetite and sleep. There's little treatment available, according to the NIH. Some research is being conducted to investigate a possible autoimmune connection, according to the Stanford University Center for Narcolepsy, which could lead to more treatment options.
Exploding Head Syndrome
Despite how troubling the name sounds, this phenomenon isn't actually dangerous -- although it can certainly be disruptive to sleep. People with Exploding Head Syndrome experience a loud bang sound, like a gunshot, cymbals crashing or a bomb exploding, the American Sleep Association explains, even though no such noise actually occurred, American Academy of Sleep Medicine spokesperson Ronald Kramer told US News. It may also be accompanied by a bright flash of light. There's no known cause (or treatment), although extreme stress and fatigue may play a roll, according to the ASA.
Fatal Familial Insomnia
This incredibly rare brain disease impairs the part of the brain called the thalamus, stripping people of the ability to sleep, US News reported. This particular type of insomnia is fatal, as its name implies: Most people with FFI die six to 32 months after showing symptoms, according to the NIH.
FFI is often also accompanied by hallucinations and eventually loss of motor and cognitive skills, according to the NIH. Passed down through families, FFI is 50 percent more likely in a child if a parent is a carrier, US News reported.
One Italian family with FFI has spoken with numerous news outlets in hopes that more awareness will lead to a cure, but there is currently no treatment. Around the world, only 27 other families have been identified as carriers of FFI, according to NBC News.