Josh Smith says it all started when he was five years old. He'd go to sleep and in the middle of the night, he'd wake up, his body rigid.

One night stood out to him in particular: He was 16. He'd just had sex with his girlfriend and it was stressing him out. Raised Evangelical, he wondered if he was going to hell for premarital sex.

He was already worried his parents might divorce; another source of profound stress in his life at the time.

He fell asleep. Then, Smith says, sometime around one or two o'clock in the morning, he woke up and couldn't move a muscle.

The bedroom was pitch black. And he had a terrifying hallucination.

"A figure appeared out of the left side of my peripheral," Smith recalls. "It was humanoid, and it was pale white. No pigment in the skin. No eyebrows, no facial hair — none of that. But the eyes were the deepest of deep. Just darkness." A hand crept into his vision until it was inches from his face.

But he still couldn't call out for help. His body was frozen.

Smith has never been diagnosed, but he thinks this was just another instance of sleep paralysis. Short Wave spoke with doctors who confirmed that Smith's symptoms are consistent with the disorder: complete immobility upon waking, a feeling of terror and occasionally, hallucinations.

But Smith says he didn't have the tools to understand this sleep disorder when he was a teenager. "I love my parents and everything, but they were not good at handling anything that wasn't 'normal.' So if you came to them with a nightmare, they would lock their door and tell you to go back to bed."

So that night? He began to call out the name of Jesus — over and over — in his head. After what Smith thinks amounted to 15 to 20 seconds, the figure disappeared.

Sleep paralysis is rooted in problems with REM sleep. There are two categories of sleep one experiences throughout the night: rapid eye movement (REM) and non-REM sleep.

Dr. Clete Kushida, a neurologist and Division Chief and Medical Director of Stanford Sleep Medicine Center, says that as a person falls asleep, their heart rate slows, muscle activity decreases and brainwave activity slows in frequency. This is non-REM sleep.

During REM, however, a person's eyes will dart back and forth, but voluntary muscles are paralyzed (think your legs, not your heart). Kushida says this stage is when about 90% of dreams occur. Brain activity is similar to when a person is awake.

It's coming out of REM sleep, when a person's voluntary muscles are still paralyzed, that a person could experience an episode of sleep paralysis.

About 75% of the time, those experiencing sleep paralysis will hallucinate. "Most of the time, we'll hear that it's something frightening," Kushida says. "But there have been instances where a person will report that one of their loved ones was there."

In fact, the hallucinations typical of sleep paralysis are frequently influenced by a person's culture and described in terms of "paranormal activity." There are descriptions from Newfoundland of an "Old Hag," or witch sitting on the person's body. In Japanese folklore, the same phenomenon is called kanashibari, which means "bound in metal." Some researchers think sleep paralysis is to blame for reports of alien abductions.

This prevalence across cultures makes sense in light of Pennsylvania researchers' systematic review of scientific studies across a 50-year period. The review estimates that 8% of the general population has one episode of sleep paralysis in the course of their lifetime. The study found that this number is variable within populations. For example, more than 30% of psychiatric patients had an episode of sleep paralysis, and the disorder is most common in adolescents.

Though researchers still have a long way to go before pinpointing exactly what causes sleep paralysis, there are some leads. In 2012, researchers at the University of Toronto manipulated transmitters and receptors in the brains of rats and were able to provoke sleep paralysis. The hope is that by studying these mechanisms, researchers will ultimately be able to understand what mechanisms in the brain cause sleep paralysis in humans.

The disorder is also common in patients experiencing narcolepsy, a sleep disorder that can cause people to wake up frequently during the night. Sleep paralysis is considered a symptom of narcolepsy.

Researchers are hoping to one day locate the genetic markers for the disorder, which has been shown to run in families.

As for Smith, he's worked to manage his own stress over the years. Managing stress is crucial because stress is pervasive for those experiencing sleep paralysis.

Dr. Kushida has a couple takeaways for anyone who may be experiencing the disorder:

1. If the sleep paralysis episodes persist, go to a sleep specialist

Physicians there can run a sleep study and make a diagnosis. The physician will likely take into account the patient's sleep history and screen for things like sleep apnea. Though, Dr. Kushida notes, "a sleep study is not necessary for diagnosis."

2. If you suspect someone is going through this, provide comfort in those moments because it is pretty scary.

Dr. Kushida says that reassurance is important for those experiencing sleep paralysis, "because in most cases it just spontaneously disappears. But if it happens repetitively, then they should see a sleep specialist or a neurologist."

These specialists can further diagnose the patient and if needed, refer the patient to a specialist who can help tackle what's triggering the sleep paralysis — be it stress or something else.

This story is a listener question episode of Short Wave. Send us YOUR questions to shortwave@npr.org and the team may answer them.

This episode was produced by Rebecca Ramirez and edited by Viet Le.