Cultural specifics can also tweak universal themes. Dr. Bulkeley and his colleagues have found that nightmares about falling through the air are common among women in Arab nations, perhaps for metaphorical reasons. “There’s such a premium in these countries on women remaining chaste, and the dangers of becoming a ‘fallen woman’ are so intense,” he said, “that the naturally high baseline of falling dreams is amped up even more.
Using brain imaging devices that are noisy and uncomfortable and less than conducive to a good night’s sleep, scientists have nonetheless begun identifying which regions of the brain are active during sleep and which are largely off-line. The brain proceeds through four stages of sleep at night, each characterized by its own pattern of brainwaves and neurochemical activity. REM sleep, when the eyes are flitting behind closed lids, is rightly renowned as the dreaming stage, with at least 90 percent of it spent dreaming. But dreams occur in parts of non-REM sleep, as well.
When slipping into REM sleep, Dr. Levin said, “the whole brain changes.” “Neurochemically, it’s like the Fourth of July,” as cortical precincts shift colors in scanning images to indicate arousal or quiescence, he said, adding, “The limbic system becomes incredibly active, much more so than when you’re awake, which is why you’re emotionally on edge in dreams.”
Blazing with particularly patriotic fervor in the limbic system are the amygdala and anterior cingulate cortex, constituting what Steven H. Woodward, a psychologist at the V.A. hospital in Menlo Park, Calif., terms the brain’s “axis of fear.” At the same time, the prefrontal cortex, seat of rational thought and critical reasoning, is on lunch break, Dr. Levin said, “which is why you can have a dream where something has 4 heads and 12 legs, and you think, ‘No problem, what’s next?’”
Also relatively tranquilized is the primary visual cortex, recipient of visual signals from the outside world. The secondary visual cortex, however, which helps process and interpret those signals, remains alert. It is here that the fabulous imagery of dreams probably arises, said Tore Nielsen of the University of Montreal, as the secondary visual cortex strives to decipher the signals ricocheting through it, many of them internally generated, and to splice them into some approximation of a coherent whole.
Other sensory and motor systems remain active in REM, including those that would normally control the arms and legs, which is why motion figures prominently in many dreams. But if you often feel frustrated, as though you can never get to where you’re going, well, you can’t.
As it happens, one vigilant player in dreaming is a small region of the brainstem that paralyzes most of the body, preventing you from physically acting out your dream. People with neurogenerative diseases that disable this brainstem disabler can end up injuring themselves during extreme dream-driven actions. Most cases of sleepwalking occur in non-REM sleep, when the body is not paralyzed.
With so much of the sleeping body and brain apparently colluding to allow us to wander safely through an ominous dreamscape of extravagant characters, most sleep scientists are convinced that dreaming serves an essential, possibly evolutionarily adaptive, purpose.
In a recent paper in Psychological Bulletin, Dr. Nielsen and Dr. Levin proposed that dreaming served to create what they call “fear extinction memories,” the brain’s way of scrambling, detoxifying and finally discarding old fearful memories, the better to move on and make synaptic space for any novel threats that may show up at the door. “The brain learns quickly what to be afraid of,” Dr. Nielsen said. “But if there isn’t a check on the process, we’d fear things in adulthood we feared in childhood.”
Ordinary bad dreams rarely recapitulate unpleasant events from real life but instead cannibalize them for props and spare parts, and through that reinvention, Dr. Nielsen explained, the fears are defanged. “A bad dream that doesn’t lead to awakening is successful in dealing with intense emotion,” he said. “It’s disturbing, but there is some kind of resolution to the extent we don’t wake up.”
By this scenario, nightmares, in allowing you to escape prematurely, represent a failure of the “fear extinction” system. “Bad dreams are functional, nightmares dysfunctional,” he said.
If you feel yourself falling, spread your arms out and learn how to fly.