Sex Sleep Eat Drink Dream (27 page)

If you doze, you lose time for work and play. If you don't doze, you lose the ability to concentrate, react quickly, and fight off infection. You also put yourself at greater risk for diabetes and obesity, high blood pressure and heart trouble.

It's enough to keep anyone awake with worry.

However, as Allan Rechtschaffen says, "The effects of sleep deprivation alone will not tell us the function of sleep." Saying that sleep is necessary to stay alert, awake, and healthy is inadequate, says Rechtschaffen. "We have not even begun to understand what it is about the physiology of sleep that is necessary to prevent the effects of sleep deprivation. Would we be happy with the conclusion that the function of eating was to prevent an increase in appetite? We need to know much more."

Some hints have come from surprising sources: the giraffe and the shrew, for instance. Jerry Siegel and others have delved into the sleep of dozens of species to probe its purpose. All animals sleep, says Siegel. There's the watchful one-eyed sleep of certain birds, which lets them scan their surroundings for possible signs of danger while they snooze. Dolphins sleep half a brain at a time, allowing them to swim and control their breathing voluntarily while getting restful slumber. First one hemisphere sleeps for a couple of hours, then the other, until the dolphin has satisfied its sleep needs.

In scrutinizing animal sleep, Siegel has found some unexpected patterns: "The amount of time an animal sleeps and the time it spends in
REM
sleep vary enormously over different animal species," he says, "even among species of the same order." This is surprising, because closely related species usually have a similar brain structure and DNA, and so would be expected to have similar sleep habits. There are some general rules, however: Herbivores, who must eat throughout the day to fill their needs, have shorter sleeps; carnivores, who can get a meal in one fell swoop, have longer ones. Omnivorous humans fall in the middle. For Siegel, the way nature has adapted sleep to the living conditions of an organism suggests that sleep's core function may be to help a creature exploit its specific ecological niche.

Whatever functions occur during sleep, says Siegel, may have migrated to the quiescent period of a twenty-four-hour day because it's efficient to perform them then. One such task is the repair of metabolic damage done to the body in wakefulness. "Sleep duration is linked to body size," Siegel explains. "The bigger the animal, the shorter the sleep." Giraffes and elephants sleep two to four hours a day; armadillos and opossums about eighteen hours. Siegel suspects this has something to do with the high metabolic rate of small animals, which generates more cell damage, requiring more of sleep's restorative effects.

Repair during sleep may be especially important for the brain. Sleep may give the brain an opportunity to mend itself and to do "housekeeping" chores, such as restocking proteins and strengthening synapses, says Siegel. To achieve these tasks, the brain must shut down so that the metabolic activity of neurons doesn't get in the way. The lower brain temperature and slowed metabolic rate that accompanies deep sleep may allow enzymes to more efficiently repair and rejuvenate cells.

Most scientists believe that sleep has more than one function and that
REM
and non
-REM
sleep each plays a different, important role. Humans and animals made to go without one kind of sleep or the other will subsequently make up the debt in that particular type of sleep. The more sleep-deprived people are, the more quickly they will sink into deep non
-REM
sleep. If deprived of
REM
sleep only, they will almost immediately pop into
REM
that is more intense than normal
REM
, with more frequent eye movements.

Siegel suspects that one key to
REM'S
work may lie in the systems of brain cells that are quieted during this sleep state. Two neurotransmitter systems that are halted during
REM
, noradrenaline and serotonin, are the ones normally active in waking, enabling body movement and heightening sensory awareness. Siegel theorizes that the shutdown of these systems maintains their sensitivity and smooth functioning during daytime hours. This may explain the uplifting effect of
REM
deprivation on mood in people with depression. Depriving the brain of
REM
sleep—that is, allowing the serotonin and noradrenaline systems to keep producing their chemicals—boosts the amount of serotonin available to cells. This is the same principle at work in antidepressant drugs such as Prozac and Zoloft.

Another school of thought suggests a more revolutionary role for
REM
: quite literally, to make up the mind.

The first time I observed
REM
sleep was when my infant daughter dropped off after nursing, her head curled into my chest, her hands spread starfish-wise. Almost immediately, I could see the little flickering eye movements of
REM.
Babies usually spend four times longer in
REM
than do adults, about eight hours every day. "Indeed, most animals born immature sleep a lot and have a higher total
REM
time at birth and throughout life," says Siegel.

Why? One theory holds that
REM
helps establish brain connections during crucial periods of development. The idea is this: At birth, the brain has far more neurons than it needs. As it matures in infancy, it prunes redundant cells and connections in the cortex to strengthen key networks. Cells that are inactive are eliminated. Babies' brains, like those of adults, need periods of deep, quiet sleep for recovery and restoration. But such sleep inactivates brain cells. So, the theory goes,
REM
steps in to keep neurons in crucial networks active in the resting baby's mind to rescue them from the shearing.

But then why does
REM
sleep continue through adulthood?

Perchance to learn.

When I was in high school, there was a fad for studying for exams overnight by listening to taped recitations of facts and figures. These were supposed to soak magically into our slumbering brains in time for morning retrieval. So-called sleep learning—acquiring new knowledge while we're asleep—has been a focus of experimentation for decades. In the 1940s, a scientist reported having eradicated the nail-biting habits of almost a third of the boys at a summer camp by admonishing them in their sleep thousands of times over the course of eight weeks. More recently, a group of Finnish researchers claimed to have taught full-term human newborns to discriminate between similar vowel sounds while they were fast asleep, which suggested to the scientists that "this route to learning may be more efficient in neonates than it is generally thought in adults."

But most scientists agree that learning during sleep—that is, actively acquiring new knowledge—is probably impossible. Certainly, attempts to teach slumbering adult subjects vocabulary or foreign languages or lists of items have failed miserably. But mounting evidence supports the idea that sleep—either
REM
or slow-wave or both—may be essential to effective learning and the formation of memories after the fact, that the sleeping brain processes, sorts, and stores information it gleaned during waking hours.

"Most of us know we need to get a good night's sleep in order to be at our best in learning a task or taking a test," says Charles Czeisler. "What many don't realize is that the sleep we obtain the night
after
learning the task is critical for learning it well, for consolidating the memory of that task."

Robert Stickgold and his colleagues at Harvard recently demonstrated that a night's sleep after learning either a visual task or a motor skill was crucial to improved performance. Subjects taught the visual task could not better their performance beyond a certain level without "sleeping on it." Those taught the motor task showed a 20 percent increase in speed after a night of sleep, while an equivalent period of wakeful time offered no benefit.

Some years ago, scientists found that the same parts of the brain activated while people learned a task were again activated during
REM
sleep. It's called the replay phenomenon. One recent study showed that the precise part of the hippocampus that fired up during learning a spatial task—finding one's way through a virtual town—was again active during slow-wave sleep.

Perhaps sleep allows the brain to review neural connections made during the day. Or perhaps it serves to reset their strength, restoring the brain's homeostasis, posits neurobiologist Giulio Tononi. In 2004, Tononi and his colleagues reported finding that the specific parts of the brain used during a daytime learning task showed heightened slow-wave activity at night. The team asked volunteers to perform a task requiring complicated hand-eye coordination before they went to sleep. The task is known to demand activity in a particular region of the right parietal cortex of the brain. After training, the subjects slept while their brain activity was monitored with MRI imaging and EEG readings from 256 electrodes on their scalp. The results showed that during post-training sleep, slow-wave activity increased only in that region thought to be activated by the task. Moreover, the more deeply these brain parts slept, the more the subject improved in performing the task the next day.

My favorite new finding suggests that sleep may not just rest our brains or reinforce what we know, but may help to create fresh insight. A team led by Ullrich Wagner of the University of Lübeck in Germany offered experimental evidence that sleep not only consolidates recent memories; by changing the way memories are structured in the brain, it may also foster breakthrough thinking and creative solutions to intractable problems.

In 2004, Wagner's team decided to investigate the creativity theory by testing subjects on a challenging task of logic. The subjects were asked to transform a series of strings of eight digits into new strings using two rules about pairing the digits, and then to deduce as quickly as possible the last number in the new sequence. They were not told of a hidden third rule, which was a quick shortcut to the answer. The subjects were trained in the task, tested, then given an eight-hour break before being retested. One group slept during the break; another remained awake. Of those who slept, 60 percent spotted the shortcut—more than twice as many as those who stayed awake. The use of the hidden rule could not have stemmed from practice, Wagner concluded, but must have arisen during sleep, when elements of the task learned in the training were rearranged. During sleep, as our brain shifts our memories from fresh to permanent status, it reorganizes them; this memory shuffling facilitates new insights.

So, says Wagner, the best way to deal with a problem may be to mull it over before going to bed and then just sleep on it.

The annals of literature hold numerous tales of insight emerging during or just after sleep. The poem "Kubla Khan" is said to have come to Coleridge in a dream. Robert Louis Stevenson claimed to have dreamed pivotal scenes in his novella
The Strange Case of Dr. Jekyll and Mr. Hyde.
"I had long been trying to write a story ... on that strong sense of man's double being," he wrote. "For two days I went about racking my brains for a plot of any sort; and on the second night I dreamed the scene at the window, and a scene afterward split in two, in which Hyde, pursued for some crime, took the powder and underwent the change in the presence of his pursuers."

Science also holds stories of discoveries arising from somnolent states. Dmitri Mendeleev dreamed of a periodic table where all of the elements fell into their proper places. Breakthrough dreams on two consecutive nights aided Otto Loewi in envisaging the design for experiments that would reveal the chemical transmission of nerve impulses. One night the great chemist Friedrich August Kekulá was brooding over the mysterious structure of aromatic compounds such as benzene, which occur in fragrant oils and spices. He turned his chair to the fire and dozed. "The atoms were gamboling before my eyes," he wrote. "Long rows ... all twining and twisting in snake-like motion. But look! What was that? One of the snakes had seized hold of its own tail, and the form whirled mockingly before my eyes." Here, in the dream of a ring, was the solution to the structure of benzene.

Some would dispute these stories of discovery through sleep or dreams, arguing that the eureka! moments originated not in sleep but in that drowsy state of
dormiveglia,
when the conscious mind is still at work—or in Coleridge's case, in an opium-induced stupor. But the possibility that dreams might promote creative thought makes sense to me. I think back to that metaphoric dream I had a decade ago, of plunging headfirst into a bed of mud, which resolved my debate over medical school. Sometimes, in the face of a difficult decision, rational thinking, itemizing pros and cons, just doesn't work. Sleeping on it does, perhaps by creating a kind of practice run of various scenarios and testing our emotional response to them. Or perhaps by tying together unlike elements that we would never think to link while awake. Or perhaps just by giving us rest from our rational mind.

13. HOUR OF THE WOLF

I
T'S
2
A.M.,
long past lights-out, but you murmur and toss, inching through darkness toward a night of no Morpheus. Who else is awake at this ungodly hour? Truck drivers, oil riggers, hospital doctors, pilots and air-traffic controllers, bakers, musicians, late-night partygoers heading home, and many, many of the elderly.

Aging sabotages both sleep and circadian rhythms. When William Dement and Mary Carskadon studied the sleep of healthy men and women ranging in age from sixty-five to eighty-eight, they found that most experienced frequent "microarousals"—the opposite of microsleeps. These brief awakenings may last only a few seconds, but they can occur between two hundred and a thousand times a night, badly disrupting deep sleep. This loss of deep sleep actually begins in midlife. Between the ages of thirty-six and fifty, less than 4 percent of our sleep time is of the deep variety, roughly a fifth of what we enjoyed in early adulthood.