Every night we lie mostly paralyzed and unconscious, snuffed out except for brief flares of Technicolor quasi-psychosis we call dreaming. Why?
For most of my life I was an unrepentant sleep snob. When I lay my head upon the pillow, sleep and I were one. I used to test my talent for flowing fluidly in and precisely out of sleep: just for the fun of it, I would say as I lay down, “Okay, brain, wake me up at 6:18 a.m. precisely” (or 6:21 or 7:02). And I could do it. I had a gift for sleep—and only the most cursory pity for my childhood best friend, who suffered the agonies of insomnia.
It seems my comeuppance was long overdue. Last winter, for seven wretched months, I hardly slept. A perfect storm of problems left me staggering zombie-like through the days. Even the ability to nap was snatched from me. I would wake my boyfriend in a fit of jealous pique at night, begging for peanut-oil massages to calm me down but also perversely unable to tolerate his seemingly comatose body nearby. Needless to say, there were a few four a.m. fights—he didn’t understand the logic that two sleepless people were better than one.
The season of hell has mercifully passed and left in its wake a new appreciation for the beneficence of sleep. How do I perform this nightly act of magic? And why must I? On those nights last winter, I’d leave my hated bed and Google endlessly about sleep, oddly soothed by the misty silence that permeates New York City at the witching hour. I learned that sleep is as old as complex life itself — a dinosaur skeleton was recently discovered in a sleeping position like a bird’s. Dolphins sleep one brain hemisphere at a time so they can keep swimming; armadillos and bats sleep 18 hours a night and giraffes only two; platypuses spend 10 hours a day in what is known as paradoxical, or rapid-eye movement (REM), sleep. Even fruit flies seem to sleep, alternating periods of deep rest with activity. Such creative, convergent evolution reveals that sleep is so necessary that nature will invent it again and again, even if she varies the details.
The less I slept and the more I craved sleep, the more strange the act of sleep itself seemed. Why was there such a difference between lying there resting (but not sleeping) and being “out like a light”? “Sleep is a beautiful place to study the border between consciousness and unconsciousness,” says psychiatrist Robert Stickgold, head of the Center for Sleep and Cognition at Harvard Medical School. “You ring an alarm in front of someone who is sound asleep and [he wakes] up,” he says. “If you understood what changed in [his] brain in that split second, you’d understand consciousness itself. I study sleep in order to understand waking.”
I came away from my bout of insomnia in awe of sleep, never to take it for granted again. It’s the most dramatic modification of consciousness we undergo, and it happens to all of us every night. Here’s a bit of what I learned.
The Sleeping Brain Is a Chatterbox
Every night we lie mostly paralyzed and unconscious, snuffed out except for brief flares of Technicolor quasi-psychosis we call dreaming. Why? A growing number of neuroscientists suspect the primary function of sleep is to preserve brain plasticity—the brain’s ability to change in direct response to experience. Neurons all over the brain “talk” to each other while we sleep, strengthen certain connections, pare down others, and discard what is deemed unnecessary. “At night,” says neurobiologist Terrence Sejnwoski, head of the Computational Neurobiology Laboratory at the Salk Institute in La Jolla, California, “the sleeping brain is untethered from the world and is a whirlwind of activity. Neurons are forming coherent patterns that without sleep would not be nearly as extensive, robust, stable, and flexible.”
Rest and hibernation both lack sleep’s restorative powers. But hibernation is really interesting because of what it reveals about sleep and neuronal growth. In deep hibernation, the brain temperature of Golden-mantled Ground Squirrels can plummet to nearly zero degrees centigrade. During their half-year of hibernation, the squirrels periodically heat up and enter “the most intense sleep period that you ever see,” says biologist Craig Heller of Stanford University. As they enter sleep, their EEG patterns show a massive sleep deficit, the precise kind of slow-wave pattern you’d see in an animal that has been extraordinarily sleep deprived. Studying the brains of these squirrels, Heller found that while hibernating, there was a tremendous dying-off in neuronal and synaptic connections. When the squirrels warmed up and slept, neurons blossomed rapidly in the brain, like flowers in time-lapse photography. Amazingly, synaptic complexity was restored within hours. Sleep may truly foster neuronal connections.
Do We Sleep to Learn?
Why do we need all that pruning and strengthening of neurons? Because our brains are constantly learning, and the more they learn, the better they can predict—and prediction is necessary for organisms that move, says Rodolfo Llinas, chairman of the Department of Physiology and Neuroscience at New York University School of Medicine, and author of I of the Vortex. Plants don’t have brains, says Llinas, because they are stationary. In fact, in a novel twist of evolution, a creature called the sea squirt moves freely until, as an adult, it attaches itself to a rock. At that point, it eats and digests its own brain. The reason, theorizes Llinas, is that once it’s fixed in place, it no longer needs a brain to predict anything.
After learning something, if you don’t snooze, you lose. One 2000 study by Stickgold and colleagues assessed the impact of sleep on performance by testing some individuals after a night of sleep deprivation, and then again a few days later after they’d caught up on their sleep. Participants who slept the night after training performed the task better the next day and kept improving over the next three days, without further training.
Those who were sleep deprived that night didn’t improve, not even when allowed to catch up on their sleep for two nights.
Sleep markedly improves our unique “eureka!” insights, according to an ingenious 2004 study by Jan Born and colleagues at the University of Lübeck’s (Germany) Department of Neuroendocrinology. Sixty-six individuals were asked to solve a sequence of problems. There were three groups: those trained in the morning, who then stayed awake for at least eight hours; those trained in the evening, who were sleep-deprived for at least eight hours; and those trained in the evening, who then went to sleep. Amazingly, sleep more than doubled the probability of gaining a eureka insight into a hidden rule that would solve the problems.
Sleep Is Not One But Many
Though we experience sleep as an absolute—we “fall” into it and “wake up” out of it—different parts of the brain seem to sleep more intensely than others. Excite a neuron by day, and it reactivates at night. In 2000, Alexander Borbely, a sleep scientist at the University of Zurich, showed just this by shaving whiskers on one side of a rat’s face. Rats use their whiskers to orient themselves spatially, and each whisker is associated with a tiny column in the cortex. Borbely found that the part of the cortex associated with the intact whiskers had stronger EEG patterns during subsequent non-REM sleep, as if those neurons were sleepier because they had been more active. Other studies in humans have shown something similar: small areas of the brain stimulated during the day sleep “harder” at night (more slow-wave sleep activity).
Giulio Tononi, M.D., PhD at the University of Wisconsin, is also zooming in on sleep at the most fine-tuned level: our genes. He and colleague Ciara Cirello recently discovered genes that are highly active only in sleep. So what are these “sleep” genes doing? Tononi believes the genes help quiet down synaptic activity. “During waking,” says Tononi, “you’re going to change a lot of circuits in your brain and strengthen many synapses. The total strength of a synapse is called its weight. So you could say by the end of the day your neurons are heavier than they were in the morning. We believe that sleep, and the genes that regulate it, downscale these synapses, and help them shed some of their excess weight. Our hypothesis, which is most likely wrong in some of its details but hopefully will stimulate further experiments, presents yet another way of looking at sleep. It says there is a cost to all the learning you do while awake. It says that sleep is the price we pay for such an expensive and fantastic learning instrument as the brain.”
And though most of us regard a good night’s sleep as the ability to slip under the covers and wake refreshed eight hours later, that kind of “perfect” sleep may not even be natural. Before we lit up the night with fluorescent and incandescent bulbs, sleep was often broken into a first and a second sleep. After sunset we’d sleep for about four hours, wake for a few hours, and then go back to sleep for another four hours. According to written records, diaries, court documents, and even literature from as early as the first century, after their first sleep, people would have sex, do household chores, or even visit friends. And this pattern may be deeply natural to us: a study in the early 1990s exposed eight healthy men to conditions that emulated winter: natural and artificial light for 10 hours each day, and then darkness for 14 hours each night. They began to sleep in two bouts of about four hours each, broken by up to three hours of quiet wakefulness—just like Ben Franklin, who took “cold-air baths” while reading naked in a chair in the silence between his sleeps.
How come, when I couldn’t get a good night’s sleep for days on end, I felt like I was coming down with a horrific flu? It turns out that sleep is highly correlated with shifts in immune and endocrine function. Rest simply does not do the trick. That’s surpassingly strange, since during sleep our metabolism only slows down by about 10 percent. And yet, sleep-deprived rats in the lab eventually die, beset by skin lesions and infections, unable to regulate body temperature, gorging themselves on food while rapidly losing weight. A six-year study by the University of California and the American Cancer Society of over a million adults found a significantly higher mortality rate among those who slept less than four hours or more than eight hours a night. Another study of 10,000 adults found that those who sleep less than seven hours a night are significantly more likely to be obese. Too little sleep is correlated with high blood pressure, depression, heart disease, and blood sugar problems. Just three nights of disrupted sleep wreaks havoc with the body’s ability to control blood sugar levels.
According to neuroendocrinologist Jan Born, sleep may help the immune system remain flexible and responsive. Born and his colleagues found that a night of sleep doubled the amount of antibodies individuals formed to the hepatitis A vaccine. Nineteen healthy individuals who had never been infected with hepatitis A were given the vaccine. Half stayed awake that night, while the other half slept. Four weeks later, the sleepers had twice as many antibodies as those who’d been sleep deprived for that one single night.
Born was both surprised and excited. “The immune system has to develop a long-term strategy to a new antigen, which requires memory,” he says. “Perhaps sleep allows our immune system to form long-term memories and eureka insights, much like our brain forms them during sleep.” If sleep modulates learning in both our brain and our immune system, it is truly a marvel of evolutionary adaptation.
To wax a bit philosophical here, perhaps the essence of sleep (and its opposite, waking) has to do with oscillation. Everything alive oscillates. Life ebbs and flows in harmony with the rotation of our planet and is entrained by circadian rhythms. The circadian genes that control rest and activity in the fruit fly are almost perfectly preserved in mice and, largely so, in humans. In fact, all things great and small seesaw between quiescence and activity—even neurons will start oscillating, seemingly magically, if immersed in the proper ionic solution, according to research by neuroscientist David McCormick of Yale University. And so, we must wake, and we must sleep. It’s writ in the stars.
Jill Neimark is a science journalist, novelist, and poet. Her latest book is Why Good Things Happen to Good People (Bantam, 2008), whygoodthingshappen.com.