They say that when the twin towers of the World Trade Center came crashing down and airliners plowed into the Pentagon and a Pennsylvania field, all Americans were traumatized.
The proof, if any was really needed, lay in their dreams.
For days after 9/11, people across the United States reported unusually intense dreams — not necessarily about towers or airplanes, but the images were more highly charged and linked to emotions such as fear and terror than before the attacks.
"What they were dreaming," said Ernest Hartmann, a Tufts University School of Medicine psychiatrist, "is that they were overwhelmed." Hartmann, who has studied dreams for more than 30 years, believes he may be the first person who has systematically analyzed the link between a national trauma and the dreams it caused.
Scientists, whose endless quest for objective facts is often trumped by the subjective nature of dreams, have yet to develop a common theory on the purpose or meaning of dreams.
In just the past six years, researchers at the University of Pittsburgh, the National Institutes of Health and the University of Liege, Belgium, have used advanced imaging techniques to peer deep within the brain of sleeping subjects to see which areas are most active during dream episodes.
They've found that when dreaming is at its peak, the sleeper's brain is highly aroused, almost as if it were on emotional overdrive.
No device yet allows researchers to look over a dreamer's shoulder to evaluate the content of dreams, so scientists like Hartmann still must rely on the recollections of dreamers, which are necessarily incomplete, hazy and sometimes self-censored. In his case, he gathered dreams from 16 people around the country, culling dreams from 10 days before and 10 days after 9/11.
Sigmund Freud argued a century ago that dreams are highly symbolic and could be interpreted to reveal a person's hidden desires, but few scientists today agree with that.
As suggested by the response to 9/11, these nocturnal experiences engage the emotional center of our brains and may play an important role in regulating our emotions. Dysfunctional dreaming, on the other hand, appears to play an important role in post-traumatic stress disorder and may be involved in depression as well.
Other evidence suggests that dreams may aid in learning new mental skills and in sorting and consolidating our memories. It now seems clear that dreams do have a function and are not just hazy, confused meanderings within a groggy mind.
"What are dreams for? That's like asking, 'What is waking for?' " said Rosalind Cartwright, chairman of psychology and a sleep disorders specialist at Rush University Medical Center in Chicago.
Dreams are the mind's work.
One theory is that REM — or rapid eye movement sleep, when the most intense dreaming occurs — somehow resets special brain mechanisms and resensitizes the brain to certain neurotransmitters, the chemicals that brain cells use to communicate with one another. REM sleep, discovered just 50 years ago, appears to be the only time when the brain stops secreting serotonin, a neurotransmitter involved in regulating mood, and norepinephrine, which helps control heart rate, blood pressure and other bodily functions.
Neuroimaging studies show that the part of the brain that is responsible for short-term, or working, memory — the prefrontal cortex — is largely inactive during REM sleep. It's as if the brain isn't supposed to remember dreams, or as if remembering dreams isn't essential to their function.
Late-morning dreams are the most likely to be remembered, because that's the part of the night when REM sleep periods are longest and sleep itself is less deep.
But the view that dreams are simply a means of clearing out old junk no longer has much support, said Dr. Daniel Buysse, medical director of the University of Pittsburgh Sleep Evaluation Center and past president of the American Academy of Sleep Medicine. Researchers suspect that dreams and REM sleep may play a role in learning certain tasks and in consolidating memories.
Dreaming, said James McClelland, a Carnegie Mellon University psychologist, "is the opposite of forgetting."
During the day, he explained, a person's experiences are stored in an area of the brain called the hippocampus, which is active during REM sleep and replays these episodic memories before they are translated into long-term memories stored in the brain's cortex.
In animal experiments, one of McClelland's collaborators, Bruce McNaughton, of the University of Arizona, has implanted electrodes in the brains of rats, recording the activity of more than 100 nerve cells during both waking and REM sleep. Distinctive patterns of neural activity that McNaughton recorded while the rats ran a maze or a track were repeated during REM sleep.
It's not possible for researchers to wire up humans in the same way, but presumably people also relive daily experiences during their dreams and perhaps rehearse new skills, McClelland said.
Freud talked about dreams being day residue — that daily events became the stuff of that night's dreams. This day-residue effect recently was explored by Canadian researchers, who found that dreams that had a strong impact were often related to an event from the previous day. Those dreams also were the most likely to display a dream-lag effect, where an event is again incorporated into a dream a week later.
Just why a seven-day lag should occur is unclear, said the lead author, Tore Nielsen, of the University of Montreal Sleep Research Center. The delay may be part of a sorting mechanism that the mind uses to evaluate potential memories, he suggested, or it may simply mean that it takes a while to transfer information into long-term memory.
Several studies have shown that sleep is involved in procedural learning — learning how to ride a bike, drive a car, play games — but is less important for declarative memory, involved in remembering facts, said Pitt psychiatrist Dr. Eric Nofzinger. He also thinks the brain may use REM to rehearse instinctual behaviors.
One of the most intriguing theories about the purpose of dreams comes from one of the giants of the dream research field, Rush University's Cartwright. She theorizes that the dream evolves during the night; the plot line changes, and older memories are tapped.
Problems with dreaming also underlie post-traumatic stress disorder.
As Hartmann's study suggested, even a generalized trauma such as 9/11 can affect dreams. But people who suffer more immediate and intense traumas, such as someone directly involved in the 9/11 attacks or in combat, may have dreams so upsetting they disrupt sleep and make it impossible for the mind to get itself back in emotional balance, said Germain, who specializes in treating patients with post-traumatic stress.
Other researchers, such as Tufts' Hartmann, suggest dreams may play other roles, such as boosting creativity. In a dream, he explained, the mind is "hyperconnective." If there's any function that dreams don't seem to serve, it appears to be the one people assume is most important: to give the mind a rest.
To the contrary, said Cartwright: "The mind does not shut off. . . . It just changes channels."