You wake up unable to move, barely able to breathe... you feel an oppressive weight on your chest... and you sense some evil presence in the room... The old hag strikes!

A reader writes:

About a year and a half ago, I was awoken in the night by a strong, warm breeze. I could not move and could not scream. It lasted about 30 seconds and was gone. I saw nothing. Last week it happened again. I was lying in bed and again was awoken. I felt a very strong force holding me down. I could not sit up. I tried to scream for my daughter and could not get any noise to come out. I tried to hit the wall with my arm and this force would not let me. It again lasted about 30 seconds and was over. I really don't believe in ghosts and didn't see anything at all. I am just really scared and confused.

Have you ever had a similar experience? The above incident is a classic example of what has become known as the "old hag" syndrome and is one of many such letters I receive from readers each month. The victims awake to find that they cannot move, even though they can see, hear, feel and smell. There is sometimes the feeling of a great weight on the chest and the sense that there is a sinister or evil presence in the room. And like the above reader, they are often quite frightened about what is happening to them.

The name of the phenomenon comes from the superstitious belief that a witch - or an old hag - sits or "rides" the chest of the victims, rendering them immobile. Although that explanation isn't taken very seriously nowadays, the perplexing and often very frightening nature of the phenomenon leads many people to believe that there are supernatural forces at work - ghosts or demons.

The experience is so frightening because the victims, although paralyzed, seem to have full use of their senses. In fact, it is often accompanied by strange smells, the sound of approaching footsteps, apparitions of weird shadows or glowing eyes, and the oppressive weight on the chest, making breathing difficult if not impossible. All of the body's senses are telling the victims that something real and unusual is happening to them. The spell is broken and the victims recover often on the point of losing consciousness. Fully awake and well, they sit up, completely baffled by what just happened to them since now the room is entirely normal.

Confronted with such a bizarre and irrational experience, it's no wonder that many victims fear that they have been attacked in their beds by some malevolent spirit, demon or, perhaps, an alien visitor.

The phenomenon occurs to both men and women of various ages and seems to happen to about 15 percent of the population at least once in a lifetime. It can occur while the victim is sleeping during the day or night, and it is a worldwide phenomenon that has been documented since ancient times.

"In the 2nd century, the Greek physician Galen attributed it to indigestion," according to The Encyclopedia of Ghosts and Spirits by Rosemary Ellen Guiley. "Some individuals suffer repeated attacks over a limited period of time; others have repeated attacks for years."

Another example:

I am a 27 year old female and have been suffering for the past 12 or so years. It started just being unable to move, like someone was on top of me, pinning me down. And although I was trying with all my might to move or to scream, all I could do was barely wiggle my toes and faintly murmur. In the beginning it was very frightening and I would try with all my might to wake up. Upon waking I would be unable to resume sleep for at least a few hours. Now I have become somewhat used to them. Sometimes I even lie back and see how long I can take that awful, overpowering feeling. In the end, I always try to wake myself up.

Over the years this "thing" has kind of metamorphosized into a dark being, something who is doing this deliberately to me for some reason. I guess this is something that I may have invented in my head to deal with it. I am not really sure. After I got used to it, I never really questioned it. It still occurs about every 2 months or so. Sometimes once a night, other times it can happen several times in one night.

What's going on? Is there a rational explanation for these freaky experiences?

The Scientific Explanation

The medical establishment is quite aware of this phenomenon, but has a less sensational name than "old hag syndrome" for it. They call it "sleep paralysis" or SP (sometimes ISP for "isolated sleep paralysis").

So what causes it? Dr. Max Hirshkowitz, director of the Sleep Disorders Center at the Veterans Administration Medical Center in Houston, says that sleep paralysis occurs when the brain is in the transition state between deep, dreaming sleep (known as REM sleep for its rapid eye movement) and waking up. During REM dreaming sleep, the brain has turned off most of the body's muscle function so we cannot act out our dreams - we are temporarily paralyzed.

"Sometimes your brain doesn't fully switch off those dreams - or the paralysis - when you wake up," Hirshkowitz told ABC News. "That would explain the 'frozen' feeling and hallucinations associated with sleep paralysis." According to his research, the effect only really lasts from a few seconds to as long as a minute, but in this half-dream half-awake state, to the victim it can seem much longer.

In her article, "Help! I Can't Move!,"Florence Cardinal writes: "Sleep paralysis is often accompanied by vivid hallucinations. There may be a sense someone is in the room, or even hovering over you. At other times, there seems to be pressure on the chest, as though someone or something perched there. There may even be sexual attacks associated with the hallucinations. The sound of footsteps, doors opening and closing, voices, all can be a very frightening part of sleep paralysis. These are known as Hypnagogic and Hypnopompic Experiences and they are what make people dread an episode of sleep paralysis."

For all their explanations, however, the sleep experts still do not know what causes the brain to screw up like this, or why some people experience it more than others. But there are some theories:

"Episodes of paralysis can occur when the body is in any position, but happen most frequently when the sleeper is lying flat on his or her back. Intense fear is common, but sometimes other strong emotions, such as sadness or anger, are present," says Florence Cardinal in "The Terror of Sleep Paralysis."

For some, sleep paralysis is often brought about by not getting enough sleep or being overtired. Likewise, disrupted sleep schedules or circadian rhythm disturbances can produce an episode of sleep paralysis.

It is more common in people who suffer from severe anxiety or bipolar disorder. Some research shows that sleep paralysis is five times more likely to occur with people who are taking such anti-anxiety drugs as Xamax or Valium.

A study found that 35 percent of subjects with isolated sleep paralysis also report a history of wake panic attacks unrelated to the experience of paralysis.

How can you prevent sleep paralysis? According to clinical research, you may be able to minimize the episodes by following good sleep hygiene:

get enough sleep

reduce stress

exercise regularly (but not too close to bedtime)

keep a regular sleep schedule.

"For some people this may not be possible, however," says Florence Cardinal, "so instead let's look at ways to escape from the grip of sleep paralysis. The best remedy is to will yourself to move, even if it's only the wiggling of your little finger. This is often sufficient to break the spell. If you can manage it, scream! Your roommate may not appreciate it, but it's better than suffering through a long and fear-filled episode. If all else fails, seek professional help."

Sounds like good advice. The bottom line is that you really have nothing to fear, in a paranormal sense, from sleep paralysis. That old hag you feel perched on your chest may be nothing more than the anxiety of living in a stressful world.