You're asleep and dreaming, and you're about to be attacked. Your body is stricken with terror. You open your mouth to unleash a blood-curdling scream and...nothing comes out.

What does this common dream mean?

Symbolically, a dream like this could indicate there is something within the dreamer that deeply needs to be expressed, a "deep internal scream," says dream analyst Jane Teresa. By interpreting the other details in the dream, the dreamer can begin to understand what the dreamer is holding back, unable to express, or what is choking her on an emotional level.

It's worth considering what your brain may be trying to process. There are many theories about why we dream, AJ Marsden, assistant professor of human services and psychology at Beacon College, told Prevention; one theory gaining ground is the idea that dreams are the brain's way of trying to solve problems or deal with intense emotions.

"A nightmare may be our brain's way of preparing us for a particular fearful situation," she said, explaining that scary dreams may be our mind's rehearsal for dealing with the situation, or helping us to feel less afraid. A study in the journal Sleep found that both postpartum and pregnant women experienced dreams and nightmares involving their infants, while postpartum women had more intense nightmares about something happening to the baby. "Such behaviors," noted researchers, "may reflect a mother's state of maternal vigilance; they may even serve a functional role in her infant caregiving."

But unlike other common anxiety dreams, such as appearing in public, forgetting to attend class all semester, or teeth falling out of your mouth, there may be a physical explanation to a dream of screaming with no sound.

"This may not just be a dream, but may be the result of sleep paralysis," psychologist Dr. William Braun told Harper's Bazaar. "During REM (Rapid Eye Movement) sleep, the sleep cycle during in which we dream, our bodies experience REM atonia, a natural paralysis during the REM cycle. Physiologically, we are paralyzed during REM sleep to prevent our bodies from acting/moving while we dream."

In other words, this is nature's way of keeping us safe from acting out our dreams — running, punching, and risking physical injury. But people who experience sleep paralysis wake before the REM cycle is complete, Braun explained, temporarily sensing that immobility.

"This space between sleep and waking can be experienced as a dream. It can also be experienced in the waking state as being unable to move, talk, and in some cases breathe."

And that can feel scarier than a nightmare.

What causes sleep paralysis? Since it mostly happens as people are falling into or coming out of REM sleep, researchers believe sleep paralysis is caused by a disturbed rapid eye movement cycle. Somewhere between 25 and 50 percent of Americans have had sleep paralysis at least once, WebMD reports.

There's a correlation between sleep paralysis and narcolepsy, and sleep experts think it may be partly genetic. If you've recently been under a ton of stress or had an erratic sleep schedule—like pulling an all-nighter or taking a red eye—that can contribute to sleep paralysis, as well.

The takeaway

There are things you can do to prevent sleep paralysis, however, sleep specialist Clete Kushida advised WebMD:

Skip the nap

"Nappers seem more prone to sleep paralysis than non-nappers unless the nappers always sleep at the same time each day."

Get as much sleep as possible.

"There seems to be some evidence that people who are sleep deprived enter REM very quickly, which means they're still awake as their body gets paralyzed."

Don't sleep on your back.

Sleep experts have found a correlation between sleeping in a supine position and being vulnerable to sleep paralysis.