Dreams: Night School

Step 2: After several mostly dreamless nights, the creature is subjected to a virtual decathlon of physical ordeals designed to test its survival behaviors. Every rat is born with a set of instinctive reactions to threatening situations. These behaviors don't have to be learned; they're natural defenses—useful responses accrued over millennia of rat society.

The dream-deprived rats flubbed each of the tasks. When plopped down in a wide-open field, they did not scurry to the safety of a more sheltered area; instead, they recklessly wandered around exposed areas. When shocked, they paused briefly and then went about their business, rather than freezing in their tracks the way normal rats do. When confronted with a foreign object in their burrow, they did not bury it; instead, they groomed themselves. Had the animals been out in the wild, they would have made easy prey.

The surprise came during Step 3. Each rat was given amphetamines and tested again; nothing changed. If failure to be an effective rat were due to mere sleep deprivation, amphetamines would have reversed the effect. But that didn't happen. These rats weren't floundering because they were sleepy. Something else was going on-but what?

What Dreams Are Made Of

Dreaming is so basic to human existence, it's astonishing we don't understand it better. It consumes years of our lives, and no other single activity exerts such a powerful pull on our imaginations. Yet central as dreaming is, we still have no idea why we dream. Freud saw dreams as convoluted pathways toward fulfilling forbidden aggressive and sexual wishes; frightening dreams were wishes in disguise—wishes so scary, he believed, they had to transmute themselves into fear and masquerade as nightmares.

Later came the idea that dreams are the cognitive echoes of our efforts to work out conflicting emotions. More recently, dreams have been viewed as mere "epiphenomena"—excrescences of the brain with no function at all, the mind's attempt to make sense of random neural firing while the body restores itself during sleep. As Harvard sleep researcher Allan Hobson puts it, dreams are "the noise the brain makes while it's doing its homework."

"There's nothing closer to a consensus on the purpose and function of dreaming than there's ever been," says Deirdre Barrett, a Harvard psychologist and editor of the forthcoming The New Science of Dreaming. Indeed, no theory has been able to reconcile the findings of various subdisciplines of dream science. Until now.

Finnish psychologist Antti Revonsuo believes the marooned rats lost their ability to defend themselves not because they were exhausted but because they were robbed of their dreams. Dreams, he contends, are a training ground in which animals and people alike go over the behaviors that are key to their survival. Prevented from dreaming, the rats were unable to rehearse their survival behaviors. In other words, they were defenseless because they were out of practice.

A Theater of Threats

Say you're in a fight and somebody wraps his arms around you from the front, pinning your arms to your sides—a bear hug. Most people reflexively stiffen their body. But this is actually the worst thing to do; making your body rigid makes you easier to lift—and lets your assailant pick you up and drop you on your head, or worse, haul you off somewhere.

Better to bend your knees and lower your center of gravity so you're harder to lift. You're then free to punch your aggressor's testicles, claw the skin on his back, kick out his knee, stomp his foot, even bite his neck—unappetizing options, but effective against even the biggest thug.

The difference between the typical and optimal response could save your life. But making such a reaction swift and automatic takes practice. It's the reason martial arts students drill their movements over and over. Frequent rehearsal prepares them for that one decisive moment, ensuring that their response in an actual life-or-death situation is the one they practiced.

Dreams may do the same thing. A dream researcher at the University of Turku, in Finland, Revonsuo believes that dreams are a sort of nighttime theater in which our brains screen realistic scenarios. This virtual reality simulates emergency situations and provides an arena for safe training. As Revonsuo puts it, "The primary function of negative dreams is rehearsal for similar real events, so that threat recognition and avoidance happens faster and more automatically in comparable real situations."

Faced with actual life-or-death situations—traffic accidents, terrorist attacks, street assaults—some people report entering a mode of calm, rapid response, reacting automatically, almost without thinking. Afterward, they often say the episode felt unreal, as if it were all a dream. Threat simulation, Revonsuo believes, is why.