Imagine waking up in the night and being unable to move.

So you lie there for what seems like hours, trying to wiggle your fingers or toes, but you are paralyzed.

You want to call out for help, but you can’t draw a deep enough breath to make a loud sound.

Eventually, you’re able to move a little, and then your whole body begins to respond again.

Scary, huh?

Weird, too.

But it happens to people all the time.

It’s called sleep paralysis, and it typically occurs at the very beginning or end of sleep. The experience lasts only a few minutes at the most, and there’s no harm done — aside from the fright.

“It’s terrifying the first time it happens,” said Dr. Barbara Phillips, director of the Samaritan Sleep Center and chairwoman of the board of the National Sleep Foundation.

Phillips said in an e-mail interview that sleep paralysis happens as the body is coming out of REM — or rapid-eye-movement — sleep.

“During non-REM sleep, our brains are ‘turned off’ but our bodies can be active,” she said. This is when people experience sleep disturbances such as tooth-grinding or sleepwalking.

“In contrast, our brains are very active (probably as active as when we are awake) during REM sleep, but we are actually paralyzed,” she said. Researchers think that’s what keeps us from acting out our dreams.

“With sleep paralysis, the paralysis that is normal during REM sleep intrudes into the waking state for one reason or another,” she said.

Kathryn Hansen, director of the St. Joseph Hospital Sleep Wellness Center, put it this way: “The brain wakes up before the body wakes up.”

Sometimes, sleep paralysis is accompanied by hypnagogic hallucinations, or “waking dreams,” Phillips said.

In many such cases, people think they see a dark or menacing figure in the room with them, or they hear a strange sound but can’t pinpoint the source. Some researchers have hypothesized that people who report alien abductions are experiencing sleep paralysis in conjunction with such a hallucination.

The experience of sleep paralysis combined with a hallucination “can be very intense,” said Dr. Kevin Nelson, a University of Kentucky neurologist who has studied the correlation between sleep paralysis and near-death experiences. “They may feel like there’s a pressure on their chest, that they can’t breathe. They may feel like they’re dying.”

Nelson said episodes of sleep paralysis are “a very common thing,” but it’s difficult to pinpoint exactly how common.

“The striking thing is, people don’t talk about them,” he said.

In some cultures, there are myths to explain the experience, or words used to describe it. In those places, Nelson said, it is more frequently reported.

The Japanese have a linguistic term, kanashibari, for the experience; in Newfoundland, it is described as a visit from “the old hag.”

“In some cultures it’s very well recognized,” he said.

Phillips said as many as 25 percent of people might be affected by sleep paralysis at some point in their lives, and investigators at Stanford University have suggested that as many as half of college students experience it.

Nelson and other medical professionals who deal with sleep disorders said they sometimes see patients who are disturbed by the paralysis but have not talked to anyone about it because it seems so strange.

“They’re not alone,” Nelson said. “They’re not weird because they have it.”

People are more likely to experience sleep paralysis, the experts said, if they are undergoing sleep deprivation, work odd shifts or have erratic sleep schedules. Hansen said it also can come with stress or anxiety.

People who are in withdrawal from alcohol or drugs that can suppress REM sleep, such as antidepressants, also can be predisposed to the experience. For example, Phillips said, a person who misses a dose of antidepressant medication might be at risk.

“It really just kind of correlates to lifestyles,” she said.

The “classic example is the college kid who parties hard during spring break, and wakes up on the beach unable to move,” Phillips said. That person has deprived himself or herself of sleep, gotten onto an odd sleep schedule and drunk to much — all three of the risk factors.

Sleep paralysis and hypnagogic hallucinations are generally harmless, but the sleep experts said they can sometimes be associated with narcolepsy.

In most cases, though, Hansen said they are a sign that the person needs “to develop some good sleep habits,” such as decreasing caffeine intake before bedtime, getting regular exercise and going to bed and rising at the same time each day.

“If sleep paralysis and extreme daytime sleepiness persist even with adequate, appropriate sleep, it’s time to see a doctor,” Phillips said.

Hansen said that once, when she knew she hadn’t gotten enough rest, there was a moment when she couldn’t move or speak as she was waking up from a dream.

Sleep paralysis wasn’t frightening to her, though.

“I laughed,” she said. “Now I know how to describe it.”