Probing Question: What is the biology behind dreaming?

man sleepingAdriane Lake

You're flying through the air, the wind in your face. Or you're being chased by a team of menacing bad guys. Or perhaps you're chatting with a long-dead family member.

Though these adventures can feel incredibly real, they are, in fact, the stuff of dreams. Dream scenarios range from the calm and mundane to the surreal and dramatic—and certain themes, such as falling or being chased, seem to be universal.

People love to guess at what their dreams symbolize, but nobody knows definitively what psychological function they play, says Edward Bixler, professor of psychiatry at Penn State's Milton S. Hershey Medical Center. What we do know more about is the biology of dreaming. "To understand the dreaming state," notes Bixler, "you need to understand the basic, underlying physiology."

Sleep is divided into two states, he explains: rapid eye movement sleep (REM) and non rapid eye movement (NREM). During about eighty percent of your sleeping hours, you are in increasingly deep stages of NREM sleep, during which time your heart rate and breathing become slower and more regular and "your cerebral cortex—the outer shell of the brain—is dominated by high voltage, slow-wave activity."

The cerebral cortex is the largest structure in the brain and is associated with thought and action. "During our waking hours," says Bixler, "the cortex is actively involved and its electrical activity is low voltage with very fast waves,"—the opposite of its activity during NREM sleep when it is not actively engaged.

Dream research—which relies in part on electroencephalographic (EEG) recordings of brain waves—shows a very different pattern during REM sleep, which gets its name from the rapid eye movements that are present during this phase. "The cortex is in an awake-like state," Bixler explains, "dominated by low voltage, fast-wave activity." REM sleep is believed to be the most active period for dreaming, during which time the sleeper is more likely to have irregular breathing and heart rate and involuntary muscle jerks.

However, no matter how real that dream about skydiving may be, we don't typically jump off the edge of the bed while sleeping. Why not? The top of the spinal cord in the brain stem intercepts the commands for movement that we receive from our dreaming minds and blocks them from being received by our motor nerves, describes Bixler. "During REM sleep, cortical activity is involved but we experience a kind of general muscle paralysis that inhibits motor activity," he adds. "Some have argued that this will protect us from acting out a dream."

REM—discovered in the 1950s by University of Chicago researchers Eugene Aserinsky and Nathaniel Kleitman—"was originally established as the dreaming state by waking individuals up during different phases of sleep," says Bixler. "An awakening during NREM will result in a report of mentation—the memory of any cognitive activity taking place before waking up—about 20 percent of the time," Bixler observes. "When a subject is awakened during REM sleep, they'll report mentation about 80 percent of the time."

When asked to report what they were dreaming about, the difference between those in REM and NREM sleep was dramatic, says Bixler. "A NREM report would be a very detailed description of a scene in color," he explains, "but the REM-related report would focus on the action in the scene. In other words, it's like comparing a still picture to a movie."

Dream research continues to evolve. Says Bixler, "Further work has identified the role of the ponto-geniculo-occipital (PGO) wave, an electrical impulse that originates in the brain stem, where very basic functions such as breathing and heart rate are controlled." The PGO is now thought to be responsible for generating the visual images that occur while we sleep.

What's more, Bixler notes, "If the PGO wave generates an image during NREM sleep, there will be no further manipulation of it by the cortex." If, on the other hand, the stimulus arrives at the cortex during REM, "the cortex will act on it and generate perceived action—what we understand as a dream."

While the scientific explanation may seem to drain the mystery from dreaming, Bixler concedes that there's much we still don't know, such as why certain images are generated. "The content of dreams is influenced by recent events and experiences and not controlled by reality," he says.

Don't recall dreaming recently? Though we don't always remember it, everyone dreams every night. "It appears," says Bixler, "that one needs to awaken immediately following a dream in order to consolidate the content. Because most of us are woken up by an alarm clock, this awakening will be out of synch with the REM period and we will probably not recall it."

Edward Bixler, Ph.D., is professor of psychiatry in the Penn State College of Medicine. He can be reached at

Last Updated April 30, 2007