THURSDAY, Dec. 16 (HealthDayNews) -- Your wildest dreams probably start soon after your head hits the pillow.
A new study finds that more aggressive, emotionally charged dreams tend to occur in the early, rapid-eye-movement (REM) period of sleep, whereas deeper, non-REM slumber encourages gentler, kinder dreaming.
The finding that the brain compartmentalizes dreams into two separate sleep periods may also put to rest the theory that dreams are no more than a meaningless rehash of random images, according to the researchers.
"That's been a very dominant theory for the past decade, and if our study gets replicated, then the hypothesis that dreams are random and without purpose is refuted," said lead researcher Patrick McNamara, a professor of neurology at Boston University School of Medicine.
The findings will appear in the February issue of Psychological Science.
To sleep, perchance to dream: Although the two are fundamental human functions, the debate continues as to the exact purpose -- if any -- of dreaming.
In their study, McNamara's team decided to look at whether the content of dreams varied by sleep stage. To do so, they fitted 15 sleeping college students with special "NightCaps" that recorded whether they were in REM or non-REM sleep.
As McNamara explained, REM sleep occurs during the first couple of hours as you drift off, whereas non-REM sleep typically makes up the latter six hours in a normal eight-hour sleep cycle. During REM sleep, the brain remains somewhat more active than in non-REM sleep, especially when it comes to the brain's emotional centers.
As part of the experiment, the researchers woke participants during REM or non-REM sleep and had them verbally record their dreams, thoughts and feelings immediately before being roused from slumber.
"We found that non-REM and REM sleep specialize in the kinds of dreams they stimulate," McNamara said.
Specifically, dreams in which the sleeper is being angry or "emotionally aggressive" towards another person were much more common in REM sleep. In fact, these socially aggressive dreams were never found to occur during non-REM sleep throughout the entire study.
On the other hand, dreams involving friendly, non-threatening social interactions comprised 90 percent of all "socializing" type dreams recorded during non-REM sleep, the researchers report. That number dropped to 54 percent during REM sleep.
Why might the brain dream differently, depending on sleep stage? McNamara can only speculate at this point, but he said "there are a couple possibilities."
For example, dreams may be the brain's way of "rehearsing" social interactions, he said, priming people for the emotional challenges to come. McNamara pointed out that certain genes help govern aggressive or passive behaviors, so it might be possible that these genes are more or less active during REM vs. non-REM slumber.
The fact that the brain's emotional centers remain active during the early REM stage of sleep might also encourage emotionally aggressive dreams during that time, said sleep expert Richard Bootzin, of the University of Arizona.
Bootzin said McNamara's study "opens the door to lots of new questions," but he cautioned that it did have limitations.
"Remember, these were all college students, between 18 to 22 years of age," he said. According to Bootzin, adolescents and young adults tend to have less neurological impulse control than older adults, so the findings might not be replicated in older populations.
Dreams occurring in REM sleep also tend to be much longer than non-REM dreams, Bootzin added, so the findings may simply reflect differences in the emotional content of long dreams vs. short dreams.
McNamara agreed that much more research needs to be done before any solid conclusions can be made. But he remains intrigued by what the findings might mean for waking life, too.
If the brain is organizing dreams in a purposeful way, then socially oriented dreams might "constrain, shape, modulate or influence the number and types of interactions that you're going to engage in during the day to come," McNamara said.
McNamara's next series of experiments will look at just that possibility, comparing sleepers' dream patterns with the socializing they conduct during the following day.
If those experiments show a real correlation, he said, "it may mean that if you want to understand why people do the things they do in the day -- in terms of their social interactions -- one place to look is dreams."
For more on the reality of dreaming, head to the National Sleep Foundation.