Sex Sleep Eat Drink Dream (28 page)

To make things worse, aging may also derange the amplitude and the stability of our circadian rhythms. Some evidence suggests that in the elderly, the peaks of hormones such as melatonin and Cortisol, as well as body temperature and other functions, are not as high, and the dips not as low. Older people often have extreme lark-like rhythms; their body temperature troughs well before dawn, for example, and they fall asleep and wake up much earlier than younger folks.

Circadian biologists are still trying to fathom what causes these shifts. They may in part be rooted in age-related changes in the eye—the yellowing of lenses, for instance, which blocks some of the light necessary to properly set circadian rhythms—or, possibly, changes in the SCN, the master clock. Scientists know that normal aging doesn't change the size of the SCN or the number of cells it contains. But at least one new study, by Gene Block and his colleagues at the University of Virginia, suggests that aging disrupts the functioning of SCN cells, especially their ability to synchronize the clocks in tissues throughout the body.

To learn how aging might affect the body's clock genes, the team studied old rats with clock genes genetically modified to carry a flashing luciferase gene, the kind that "blinks" in rhythm with the expression of the target gene. They found that the rhythms in the rats' SCN cells were normal, but the rhythms of cells in some of their outlying tissues were phase-advanced or even absent. Because the rhythms in these peripheral cells could be restored by applying a chemical, the team surmised that the problem might lie not in the cells' clocks themselves but in the failure of the SCN to send them appropriate signals. Perhaps, then, this missed communication between the "grandfather" clock and the tiny peripheral tickers accounts for the many old souls prowling their dimly lit kitchens in the dead of night.

In other cultures, such as the !Kung of Botswana, the Efe of the Congo, or the Gebusi of New Guinea, anyone at all might be awake now. In traditional, non-Western societies, social activity and frequent interruptions are often embedded in a night's sleep, says Carol Worthman, an anthropologist at Emory University. When Worthman conducted the first study of sleep patterns across a wide variety of traditional cultures, she discovered that the Western model of a habitual bedtime and a single spell of solitary sleep is rare indeed. Among the Efe, says Worthman, virtually no one sleeps alone, and "one may routinely find two adults, a baby, another child, a grandparent, and perhaps a visitor sleeping together in a small space." Arousals are common, with the movement and noises of others and the traffic of staggered bedtimes and trips to urinate. The Efe often go to sleep and then get up later because they hear something interesting going on—a conversation or music—and want to join in. Someone may wake up at any hour of the night and begin to hum or play the thumb piano or start a dance. The !Kung frequently spend the wee hours of the morning in lively dialogue, using nighttime chat to entertain themselves, debate, resolve conflicts and disputes, and work through troubled relationships. Gebusi men hold all-night séances and other social activities.

There was a time when deep night would have seen most Westerners, too, up and about or in a state of semiwakefulness. Not much was known about past patterns of sleep or nighttime activity in Europe until A. Roger Ekirch, a professor of history at Virginia Polytechnic Institute, brilliantly remedied the ignorance. In a study of historical records from 1300 to 1800, Ekirch found numerous references suggesting that most Europeans broke their rest into two phases: a "first sleep" and a "second," or "morning," sleep. (Some early medical books recommended lying on the right side during the first sleep and on the left during the second, to ease digestion and maximize comfort.) The two sleeps were usually bridged by an hour or more of quiet wakefulness. During this interlude, people often rose and moved about or stayed in bed to converse, pray, make love, reflect on the dreams that preceded their awakening, or just let their minds drift in a semiconscious state.

The habit of sleeping in bouts is typical of many mammals. Is it a more natural pattern for our nights, perhaps dating from our deep past? Did our prehistoric ancestors, too, enjoy a midnight interlude, the dark antipode of a midday siesta?

Thomas Wehr of the National Institute of Mental Health once devised an experiment that hinted at an answer. To mimic ancient winter nighttime conditions, he asked volunteers to spend fourteen hours a day in darkness (6
P.M.
to 8
A.M.),
without any artificial light, for a period of one month. In the first few weeks, the subjects slept in one long, consolidated stretch of up to eleven hours—perhaps in catch-up from sleep deprivation. But eventually they fell into a pattern of two distinct periods, sleeping for four hours, from 8
P.M.
to midnight, awakening from
REM
sleep and staying awake for a couple of hours in quiet, "nonanxious" rest, then falling back asleep at 2
A.M.
for another four hours, until waking at 6 to start the day.

Wehr measured his subjects' temperature, hormones, secretion of melatonin, and EEG patterns and found that the chemistry of their nights differed from the norm, with higher levels of melatonin and sleep-related growth-hormone secretion throughout the night. The period of rest also had its own unique chemistry, with a dramatic rise in levels of prolactin, that hormone involved in lactation and (in chickens) brooding. This distinctive endocrinological state may have promoted self-reflection and a kind of quiet meditation, says Wehr. And the two-phase sleep pattern—waking from dream sleep into quiescent rest—may have provided a means of access to dreams that we've lost today. "It is tempting to speculate," Wehr writes, "that in prehistoric times this arrangement provided a channel of communication between dreams and waking life that has gradually been closed off as humans have compressed and consolidated their sleep. If so, then this alteration might provide a physiological explanation for the observation that modern humans seem to have lost touch with the wellspring of myths and fantasies."

Wehr suspects that our natural pattern is indeed the two-phase variety—at least during the long nights of winter—and that our current habit of consolidating sleep into a single bout is an artifact of contemporary life. "Modern humans no longer realize that they are capable of experiencing a range of alternative modes that may have once occurred on a seasonal basis in prehistoric times," he says, "but now lie dormant in their physiology." We have become clamped in a perpetual long-day/short-night pattern—and the squeeze is growing tighter.

You switch on your bedside lamp, one of the thousands of possible sources of artificial light by which we shorten our nights and deny ourselves access to dream life. Thomas Edison, more than anyone, put darkness and reverie in retreat.

For tens of thousands of years, nightfall was the signal for humans to sleep; sunrise, the signal to awaken. Sunlight was the only light source for recalibrating our internal clocks, setting them to the cycles of the day and also the season. After that came wood fires and lamps that burned animal fat and pitch and petroleum, which supplied light enough to see by but not sufficient to reset our biological clocks. Then came the invention of the incandescent light bulb in 1879, and the rapid spread of artificial light. Suddenly our species seemed liberated from the fetters of the solar cycle, able to pretend that every night was a midsummer's night. However, because our inner clocks still adhere to an ancient light-dark schedule, there are costs associated with this round-the-clock illumination—the magnitude of which we're only now discovering.

Our body clocks need darkness as desperately as they need light. In 2005, scientists at Vanderbilt University showed that constant light desynchronizes the firing of neurons that make up the SCN. In turning on lamps and lights after the sun has set, we unintentionally reset our biological clocks. Exposure to even low light levels of, say, 100 lux—similar to the ambient lighting of offices and living rooms—can affect the phase of our rhythms. Charles Czeisler's team has found that during the first hours of biological night, our circadian pacemakers are especially vulnerable. Light exposure in the late evening delays the phase of our clock, so that it acts as if sunrise comes later. Exposure in the early morning advances the clock, so it expects sunrise earlier. Light at night suppresses the production of the hormone melatonin. Even brief exposure to light in the middle of the night radically reduces the activity of an enzyme necessary to make melatonin.

Ours is the only species that lights up its biological night, that overrides its own rhythms, crosses time zones, works and sleeps at times that run counter to its internal clocks. We ignore what our clocks remember at our own peril.

Take transmeridian travel. Not long ago, I sat at a table in a small village in China with a number of local dignitaries and scientists. Before us was an array of beautiful and exotic foods, including bowls of
ni do,
bird's nest soup. The soup, as I understood, is made from the nests of swiftlets, surprisingly sturdy creations woven from strands of gummy saliva, which are simmered in chicken broth. I knew I had to try the expensive delicacy, but with spoon raised, I hesitated. It's not that I objected to feasting on gelatinous strands from a bird's salivary glands. I prided myself on being an adventurous eater, ready to sample all sorts of authentic foods. But my stomach would have none of this dish—or any other. I had arrived in China only a day earlier, and my insides felt as if they were still back in Virginia. In fact, a scientist friend later told me, they were.

When you fly halfway around the world, it is said, your soul takes about three days longer to get there. So does your stomach. Indeed, you may seem to arrive in one piece, says Michael Menaker, but clockwise, different parts of your body follow only slowly. For each time zone crossed, it can take up to a day for your systems to fully adjust to the new time. Two-thirds of time-zone travelers report the symptoms of jet lag—that muzzy sluggishness and upset stomach, daytime fatigue, trouble going to sleep at night (after eastward flight) or waking up too early (after westward flight), lapse of memory and alertness, and loss of appetite, to name a few. Travelers are often awoken in the middle of the night by the surge of hormones that signal morning. Symptoms are generally worse going from west to east, perhaps because it's easier for the body to adjust to a longer day than to a compressed one.

Menaker suspects that the discomfort and malaise of jet lag arise from a loss of synchrony between the body's master clock and its peripheral clocks, and also among these outlying clocks, as they attempt to catch up in a new time zone, each at its own pace. In one study, Menaker and his colleagues used genetically modified rats to monitor the impact of time shifts on the circadian rhythms of different organs. The results suggested that the master clock in the brain's SCN, which oversees our big rhythms, such as body temperature, gets back on track within about a day; the peripheral clocks in our tissues, however—those in the lungs, muscles, and liver, for instance—may take a week or more to catch up. When the brain signals the muscles to exercise, the muscles may not respond well, as their clock still has them in deep sleep. Likewise, my brain may say it's time for
ni do
in China, but to my Virginia-moored liver, it's still the middle of the night.

This lag time in clock modification is essential in normal life. If our inner clocks shifted instantaneously with sudden changes in light, they would be spinning forward and backward each time we entered or exited a dark room. The system is designed in such a way as to adjust easily to small, gradual changes in patterns of light and dark, such as seasonal changes in day length. "But transmeridian flight is an unnatural event for which the body is not prepared," says Menaker. "Crossing time zones causes large and abrupt shifts in the light cycle, which severely disorganize the system."

Leaping time zones occasionally is one thing. Doing it often is quite another. Kwangwook Cho of the University of Bristol was inspired by his own jet lag symptoms—disorientation and memory lapses—to look into the effects of frequent transmeridian travel. In a study of twenty flight attendants working for international airlines, Cho found that five years of long-haul travel caused memory problems and cognitive impairment. Further probing with saliva samples and brain scans revealed the possible cause: Attendants who flew over more than seven time zones with fewer than five days to recover between multizone flights showed boosted levels of the stress hormone Cortisol. When the body is constantly subjected to the bewildering signals of light and dark that come with long-distance travel, it grows confused about whether it's night or day and keeps making Cortisol around the clock. As we know from studies on chronic stress, Cortisol in high concentrations damages brain cells. Sure enough, the brain scans revealed shrinkage of the attendants' temporal lobe, including the hippocampus, the part of the brain so essential to learning and memory.

The swish of distant traffic sounds on a nearby highway. Who else is up in the black of night? A couple of hundred years ago, only night watchmen, gatekeepers, and perhaps the occasional cook worked through the hours of darkness. Now, some 15 percent of the U.S. workforce is made up of people who labor into the night, controlling air traffic, driving trucks, and running hospitals, fire and police stations, factories, and nuclear power stations—work that puts them radically out of phase with natural time cues.

To flesh out the effects of nighttime labor on the body, Josephine Arendt of the Center for Chronobiology in Surrey, England, has conducted extensive field studies of workers on oil rigs in the North Sea. The work on these rigs is challenging and dangerous, she says. "To get a job there, you have to pass a test where you're hung upside down in the water by helicopter. If you can get yourself out of this predicament, you can work on the rigs—though you're expected to work difficult schedules."