The man had been admitted to the hospital because of chest pains, but the intensive care unit nurses found they had a whole different problem on their hands when he fell asleep.

Illustration by Stacy Innerst, Post-Gazette Today is the fourth installment of a five-part series on the mysteries and meanings of dreams. Normally our muscles are virtually paralyzed when we dream. Today we examine what happens when that mechanism breaks down and people walk, fight and even pig out on snacks while dreaming. Click illustration for larger image. In Your Dreams Bloomfield woman never dreamed her problem was sleep eating Dream Journal: Jocelyn Hillen Dream Journal: Peter Baynes Readers share their dreams Full index to series Yesterday: Nightmares Nightmares for many are very, very scary One solution to nightmares: imagine a better ending Night terrors are rare, but normal for children Helping children cope with nightmares Resources for chidren and adults Dream Journal: Jean-Jacques Sene Monday: Dream History After 50 years, the veil begins to lift on our dreams Dream Journal: Tara Deringor Dream society seeks middle ground behind hard science and symbolism Experts trying to awaken Pittsburgh interest in dream study Sunday: Dream Science The science of dreams Dreams: From falling to failing, it's the same old stories The 12 universal dreams Dreams and the arts: PG critics pick their dream theme team Insight from a decade-long journal of dreams A dream journal 'how-to'

An older man, he was being evaluated for possible coronary artery disease, and from a medical standpoint, he was reasonably stable.

But several times during the night, he would suddenly start thrashing around in bed so violently that he was in danger of throwing himself to the floor. Hospital staff members would rush to his side, trying to restrain him so he wouldn't hurt himself or tear out his intravenous line or rip off the electrical leads to his heart monitors.

And yet the man seemed to be asleep.

The hospital staff on duty didn't know what to make of his bizarre behavior. As it turned out, it had nothing to do with his bad heart and everything to do with his bad dreams. His violent outbursts coincided with periods of Rapid Eye Movement, or REM, sleep, the period most closely associated with vivid dreams.

He was, quite literally, acting out his dreams.

The man had REM sleep behavior disorder, or RBD, one of several dysfunctions that can occur when dreams intrude on waking life.

"The hospital staff had never seen this before," said Dr. Daniel Buysse, a sleep disorders specialist, who noted that many medical personnel have had little if any experience with RBD, which was first identified in 1986. Buysse, who had been treating the man for RBD prior to his heart problems, was able to suggest medications to control the violent episodes during the remainder of his hospital stay.

Normally, the brain takes precautions to keep dreams safely where they belong.

During REM sleep, for instance, several regions of the brain are highly active, including those involved in motor control. Researchers suspect this may have something to do with the role of REM sleep in learning new skills and behaviors; in a sense, dreaming provides the brain a chance to practice a new skill, whether it's punching a keyboard or riding a bike.

But this is possible only because the brain stem has a set of cells that serve as a switch; when turned off during REM sleep, they prevent motor cortical impulses from reaching the muscles. As a result, the body is largely paralyzed during REM sleep.

In patients with REM sleep behavior disorder, however, this switch malfunctions. This would be bad enough, but the danger is compounded because RBD also affects the content of dreams, said Dr. Carlos Schenck, a psychiatrist at the Minnesota Regional Sleep Disorders Center in Minneapolis. Schenk, along with Dr. Mark Mahowald, first described RBD almost two decades ago.

The dreams often become more violent and people become more active in them. The movements may cause sleepers to throw themselves out of bed, punch or run into walls, or punch or choke their sleep partners. This can happen three or four times each night, during each episode of REM sleep.

"People can really dread going to sleep," Buysse said.

At the Sleep Disorders Center in Milan, Italy, Dr. Marco Zucconi found that among 103 patients with RBD, half reported injuring either themselves or a partner and that six out of 10 said their dreams involved fighting or other aggression.

No one knows the true prevalence of RBD, Schenck said, though it appears to occur in less than 1 percent of the population and usually in men over age 50. It sometimes can occur because of stroke damage or a brain tumor, but, he said, it often is an early symptom of a degenerative neurological disease.

"The unlucky ones will progress to develop parkinsonism," Schenck said. Ironically, these patients can suffer the tremors, muscle rigidity and slow movements characteristic of Parkinson's disease during the day, yet suffer from wild, violent movements during sleep.

In many cases, Buysse said, RBD is associated with Lewy Body disease, a form of dementia similar to Alzheimer's disease. As the degenerative disease progresses, the patient has less REM sleep and the RBD eventually subsides, he said.

These mixups between waking and sleeping behaviors also can occur during non-REM sleep, particularly in the early part of the night when sleep is especially deep.

Perhaps half of all people have at least one episode of partial arousal from non-REM sleep, which can range from cases of night terrors in small children to sleep walking or even sleep eating.

Often, these episodes are benign, but occasionally people can run seriously amuck. Buysse once had a young patient who arose screaming from sleep and started running, right through a glass door. Usually, people remember nothing about these episodes, though this young man recollected a dream in which an animal was moving slowly across the horizon.

Sometimes sleep walkers can perform amazingly complex or prolonged actions. One of Buysse's sleepwalkers got in his car and drove to the neighborhood of his childhood. Police arrested him inside a house that was once the home of a childhood friend. The occupants of the house, who had no idea who he was, described him as "out of it."

Partial arousals have been linked with murder. Perhaps the most famous case was a Toronto man with a history of sleep walking and sleeping difficulties. One night in 1987, after thinking about talking to his in-laws, he fell asleep, got up and drove 14 miles to his in-laws' home, where he choked his father-in-law into unconsciousness and stabbed and fatally beat his mother-in-law. He later said he remembered none of it.

Though charged with first-degree murder, he was acquitted by a jury after his defense team argued that the attacks occurred while he was sleep walking.

Closer to home, a Butler County man, Michael Ricksgers, argued that he, too, was in a semi-awake state when he shot and killed his wife in their bed on Christmas night 1993. In that case, however, prosecutors argued there was evidence of marital discord and Ricksgers was found guilty of first-degree murder and sentenced to life in prison.

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Much more common -- and far more benign -- are the dreaming/waking experiences that occur at the beginning or the end of the night when people are making the transition from sleep to wake or vice versa.

Most people have probably experienced "sleep starts," the body jerks that sometimes occur during light sleep. Perhaps more discomforting are occasions of sleep paralysis, which in some ways is the opposite of RBD. People will wake up, but temporarily experience the paralysis common to REM sleep because of a delay in restoring the flow of motor cortical signals to the muscles.

Perhaps more alarming are dreamlike hallucinations that can occur after waking, another consequence of carryover elements of REM sleep. These can be auditory hallucinations, in which people hear strange noises in the house, or fragments of a melody, or even sounds of people talking. The hallucinations also can be visual, with pictures on the wall that seem to move, or clothes on a chair that look like a person.

One of Buysse's patients recounts lying on his bed and awaking to see cracks in the ceiling that grew bigger and bigger until a boulder fell through and crushed him.

Then his roommate came over and it all went away.

These hallucinations can happen in lots of people and usually are of little consequence, Buysse said. But that doesn't mean they aren't upsetting.

"Some people become concerned because they think they're psychotic."

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Tomorrow: Can you control your dreams?