Sex Sleep Eat Drink Dream (25 page)

"Sleep is the most moronic fraternity in the world," wrote Vladimir Nabokov, "with the heaviest dues and the crudest rituals." Because we lose consciousness so dramatically when we doze, it was assumed for centuries that sleep shut off the brain. Slumber was a kind of passive suspension of mental activity, a dark slice out of time—an idea that held on well into the twentieth century.

Now we know that sleep is a remarkable journey of five stages repeated cyclically over the course of the night. These are richly varied states involving shifts in brain waves, body temperature, and biochemistry, in muscle and sensory activity, in thoughts and level of awareness. Though the depth and quality of the journey may differ from person to person, from age to age, the pattern is more or less the same: four or five such cycles, each about an hour and a half long, alternating between quiet deep sleep and active
REM
sleep.

The brain is hardly "absent" at any time during the passage, says Jerry Siegel, a sleep researcher at UCLA. "It actively puts itself to sleep, then self-activates during sleep. As many neurons switch on in sleep as switch off; what changes is their overall pattern of activity." While some brain areas may quiet down compared with their daytime activity, others fire up. Even during deep sleep, "when consciousness may be totally obliterated," adds J. Allan Hobson of Harvard Medical School, "the brain is still roughly 80 percent activated and thus capable of robust and elaborate information processing."

Even after the discovery of sleep's cyclical nature, the belief prevailed that sleep was mainly downtime; its purpose, to cure sleepiness. Only now are we waking up to its astonishing complexity and the range of ways it affects body and mind. In maintaining good health, says William Dement, sleep may be more critical than diet, exercise, even heredity.

Sometimes when I can't drop off, I climb out of bed and wander into my daughter's bedroom. I'm a notoriously poor observer of my own slumber, so it's enlightening to watch hers. One hand cradles her face. Her breathing is light but regular. Though it's warm in the room, she's snuggled under the covers because her body temperature is on the wane.

Did sleep come upon her like that sly fox I saw, stealing forward, pausing, stealing back? Until recently, science described falling asleep as a slow diminishment of alertness over several minutes as the brain passed from 100 percent wakefulness to 100 percent sleep. Now it appears that drifting off is not a gradual process at all but a sudden leap, a quick neural shift from consciousness of the outer world to nearly complete sensory blindness.

The shift, it turns out, is executed by a sleep switch in the hypothalamus of the brain. Baron Constantin von Economo, an Austrian neurologist, first identified the area of the brain where the switch exists in patients with encephalitis lethargica, a form of sleeping sickness that swept through Europe and North America in the 1920s. Most of Economo's patients slept excessively, twenty hours or more a day. The sleepy patients, he discovered, had lesions in the hypothalamus.

Scientists have recently pinpointed the switch, a cluster of neurons that work together to turn off the brain's arousal circuitry. It's what an electrical engineer would call a flip-flop switch, explains Clifford Saper, a neurologist at Harvard Medical School. Such switches are designed to produce "discrete states with sharp transitions and to avoid transitional states," says Saper. "This flip-flop circuit model might explain why wake-sleep transitions are often relatively abrupt (one 'falls' asleep and suddenly wakens)." People with narcolepsy behave as if their switches are destabilized, easily dozing off during the day and waking more often at night.

The cells in this switch cluster are sensitive to environmental factors such as heat, which may explain why a warm bath or a hot day causes drowsiness. But scientists suspect that the switch is primarily driven by those same two interacting mechanisms at play during the afternoon doldrums: the homeostatic pressure for sleep and the circadian alerting system. In the evening, the latter sends such a powerful alertness signal that it creates a "wake maintenance zone" between 6 and 9
P.M.,
when it's hard to get to sleep even after severe sleep deprivation—except for extreme larks. People sleep best if they go to bed two or three hours after this zone, when the pressure for sleep is heavy indeed. By this time, the circadian alerting system has started its slide toward night, and the body's master clock has signaled the pineal gland to boost its production of melatonin, telling us it's dark, time for slumber.

When sleep seized my daughter, her skeletal muscles relaxed, and the favorite stuffed panda she clutched dropped from her hand. If she had been wearing the Medusa's head of electrodes that sleep labs use to record electrical activity in the brain, the delicate, nervous marks of the pen inscribing moving paper might have recorded a shift from the alpha waves of drowsiness, like the regular teeth of a comb, to the lower-frequency theta waves of half-sleep or early sleep.

Sometimes at this early stage, the peculiar floating or falling feeling one may experience while going to sleep is interrupted by a brief spasm known as a hypnic jerk or myoclonic twitch—a quick muscular contraction in arms, legs, sometimes the whole body—which startles one back to wakefulness. This is more frequent in adults than in children, and more common in people who are nervous or overtired. Some evolutionary biologists speculate that the hypnic jerk may be a reflex left over from our arboreal ancestors—useful in avoiding a slip from a sleeping perch.

Even in the half-sleep of stage 1, my daughter would not hear my murmured "good night" or smell supper's lingering aroma of roasted potatoes. Her ability to take in signals from the outer world has slipped to nil. Her mind may wander from the focused thoughts of wakefulness—the words she learned for her vocabulary test, the impending visit by her grandmother—to more associative thinking and then to actual pictures that change rapidly, so-called hypnagogic hallucinations, which leap from subject to subject and from one outlook to another. If I nudge her awake to ask if she's asleep, she would likely deny it.

Not so after a few more minutes. As her sleep deepens, a polygraph would at first bristle with the fleeting sleep spindles (short bursts of EEG activity) and so-called K-complexes of stage 2 sleep, then shift into the longer delta waves of stage 3, and finally into the synchronized oceanic waves of deep, slow-wave, stage 4 sleep. The neurons in her brain that do their own individual thing when she's awake would begin to fire synchronously, producing those big, slow waves.

At this stage, I would have to shake her hard to wake her. Her breathing is slow and regular; her muscles are limp. Her pituitary gland may have begun releasing surges of essential hormones, including gonadotropic hormones, which play a role in the development of her sex organs, and growth hormones, which spur her cells to divide and multiply. Most of her bone growth may be taking place now. When scientists implanted tiny sensors into the shinbones of lambs and measured bone length every three minutes for three weeks, they found that 90 percent of the growth occurred when the lambs were sleeping or lying down at rest.

This stage of deep sleep may last a half hour to forty-five minutes before the spindles and K-complexes of lighter sleep again reappear. How long the stages of deep sleep last for any individual may be affected by genes. When Swiss researchers studied a gene that regulates adenosine in a group of more than one hundred student volunteers, they found that the 10 percent of students who had a mutation in the gene gleaned an extra half hour of deep sleep and reported waking up less often than those students without the mutation.

Moving through the phases of sleep is a bit like diving, plunging into deeper and deeper waters. If I sit and watch my little diver long enough, I can see her suddenly resurface. She shifts position from back to stomach or turns from side to side. Her breathing and heartbeat quicken as if she were in the throes of movement or emotion. Cells fire in the motor region of her brain, but a complex system of neurotransmitters stops these brain signals from getting through to the motoneurons that actually activate her skeletal muscles; instead, these muscles become so thoroughly relaxed they're virtually paralyzed. Only the muscles of her eyes are unaffected, and her eyeballs flit about wildly beneath her lids. Perhaps most astonishing, her brain's control over essential physiological processes diminishes, including the maintenance of her body temperature and proper levels of blood gases. A polygraph would reveal jagged theta waves, punctuated with short bursts of alpha and beta waves, as her neurons begin to fire as busily—and as individually—as they do during waking.

This is
REM
sleep, a bizarre state that in some ways resembles wakefulness more than sleep. For the next five or ten minutes, my daughter's body closes down, but her mind is off on a singular adventure of its own, seeing and hearing things that aren't there. About a quarter of a night's slumber is occupied by
REM
sleep, in four or five bouts a night, building in duration from episodes of about ten minutes at the start of a night to thirty minutes as dawn approaches. This is the time of intense dreaming. If I shook my daughter's shoulder now and asked her what was going through her mind, she might describe a dream of feckless flying or sailing down a stream of ink.

Everyone dreams, says J. Allan Hobson; any impression to the contrary is likely rooted in poor recall. We dream in both
REM
and non-
REM
sleep, but non
-REM
dreams tend to be short, fragmented, and dull,
REM
hosts the kind of delirious reverie characterized by strange, vivid hallucinations, illogical thinking, emotion, and confabulation.

Recent advances in imaging have given us a remarkably clear picture of where dreaming activity takes place—which parts of the brain are revved up and which are quiet. Silent are the regions of the prefrontal cortex known to be important in working memory, attention, and volition. The neurotransmitter systems required for these functions during waking—specifically, serotonin, histamine, and noradrenaline—are simply shut off during
REM
sleep, says Jerry Siegel; with them go insight, reasoning, and a logical sense of time. Lively are the cortical regions essential to visuospatial processing, including the hippocampus, the center for cells devoted to a sense of place and direction. This may contribute to the "virtual navigation" apparent in dreaming. Also active are the amygdala and the limbic system, both central to the feelings of anger, anxiety, elation, and fear that so often accompany a dream story.

Unlike my daughter, whose dreams—at least the ones she remembers—are nearly all sweet, I had nightmares as a child, ominous and terrifying. Tore Nielsen, of the Dream and Nightmare Laboratory at the Hôpital du Sacré-Coeur in Montreal, suspects that nightmares may in some cases arise from circadian rhythm disturbances—from
REM
activity that is phase-advanced (occurring earlier in the cycle than normal), a possibility that deserves study, he says.

In his investigation of dreams, Nielsen has found that women typically have more nightmares than men. In a group of more than a thousand university students, women reported two nightmares a month (men, about one and a half) and a higher prevalence of dreams with frightening themes. "This gender difference is quite robust," says Nielsen, "first appearing early in adolescence and measurable right into old age. The disturbing dream content may be a function of female biology—for example, monthly hormonal fluctuations—or it may be due to sociocultural influences that differentially affect women, such as traumatic experiences, depression, and sleep disorders."

The nightmare I remember best from childhood was set in a pale pink house around the corner from my home. The man and woman who lived there hovered on a balcony above the sidewalk where my mother stood, calling her name. When my mother looked up, the couple poured laundry detergent in her eyes, a steady stream of blinding white powder. I tried to scream, but the sound never left my body. And when I tried to come to her aid, I found that all my muscles were paralyzed (as they were, in
REM
sleep). I awoke with a start and lay there immobilized, heart pounding violently. It was just a dream, I told myself. But I could not flush from my mind the image of my helpless mother blinded by Cheer, and had to assure myself by padding down to her bedroom to see her peacefully asleep.

This dream was vivid and vividly remembered. Why are so many others extinguished? How many mornings I've awoken with the remnants of a dream buried deep or scattered like bones at an ancient ruin. Its residue remains, but the dream story is gone, "
REM
dreams tend to be forgotten if they are followed by extended periods of non-
REM
sleep," says Allan Rechtschaffen, the retired head of the University of Chicago Sleep Research Lab. The dreams we remember best are those arising in the final
REM
period from which we awaken.

On average, we dream for an hour and a half to two hours each night, with four or five distinct dreams. If we live out our allotted seventy-five years or so, that means we'll spend about six years vividly dreaming, for a lifetime total of some 100,000 to 200,000 dreams.

Right now I would take one brief reverie. Back in bed, the clock flicks to 12:38
A.M.
Exhaustion is doing battle with tension; my physical self, fatigued from the day's activity, is craving slumber, but my mind is alert like some wary crane. Exercise doesn't always lead to a good night's sleep, but it does tend to improve sleep patterns for insomniacs (at least as well as sleeping pills, according to some studies), and even in good sleepers it can modestly increase sleep length and depth. Some scientists suspect that the positive impact of exercise on sleep may be enhanced by the light exposure that often comes with it. A daily dose of strong natural light has been shown to have both sleep-promoting and antidepressant effects. Sedentary adults typically get about twenty minutes of daily exposure to natural light, while people who exercise get about three times that amount.