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The new system offers two great advantages: doing the right thing at the right time within the body, and also anticipating daily transitions and tailoring behavior in the environment accordingly. By carrying inside this model of the cosmos, the body keeps a step ahead of the changes going on around it, preparing for food, mates, predators, and temperature extremes brought on by day and night.

***

"Clock" seems too feeble a term for this potent circadian influence on our bodies. Though pressures are powerful to keep body conditions constant, the circadian drive causes dramatic fluctuations throughout the twenty-four-hour day. As Emerson wrote, everything looks stable until its secret is known.

Consider body temperature.

Maybe you've now popped into the shower. To wake up and get going, some people recommend running a "contrast" shower, with hot water, then cold. (This technique may do double duty, rousing others with the whooping that accompanies the cold phase.) The heat receptors just beneath your skin's surface detect temperatures up to about 113° F; the cold receptors, down to about 50°. Above or below these temperatures, pain receptors kick in. But even if you let the water get very hot or very cold, your core body temperature will not change much. (Incidentally, the number for the normal, average body temperature so well known to most of us—98.6°—is wrong. Meticulous studies involving millions of measurements have revealed that the true average daily temperature for women is 98.4°; for men, 98.1°) So skilled is the body at keeping its temperature relatively constant despite changes in the environment that a champion cold-water swimmer such as Lynne Cox can sustain her body heat in the freezing seas of the Antarctic, and a marathon runner can keep his cool in the 120° heat of Death Valley.

Our knack for holding steady our temperature and other internal conditions—called homeostasis, from the Greek words for "similar" and "steady"—may be taken for granted, but it's a remarkable phenomenon. The body maintains its internal milieu by constantly monitoring levels of glucose, carbon dioxide, hormones, temperature, even the pH of spinal fluid. These levels flutter about a set-point, or norm. An intricate and diverse network of nerves and hormones in the body senses divergences from these set-points and rectifies them by sending word to appropriate systems that can set in motion corrective mechanisms.

What we've learned lately, however, is that our set-points aren't set at all; they actually cycle in a circadian rhythm, varying according to time of day—with profound implications for how we function and the way we feel. Body temperature, for instance, typically ranges a couple of degrees Fahrenheit over the course of a day, starting out at a low of about 97° in the very early hours (so a temperature of 98.6° first thing in the morning is, in fact, a low-grade fever) and rising to as high as 99° or even 100° in the late afternoon and early evening. These temperature swings affect all sorts of bodily experiences: When our daily temperature peaks, for example, so does our tolerance of pain and our muscle flexibility, speed of reflexes, eye-hand coordination, and proofreading accuracy.

Heart rate and blood pressure also vary with time of day, along with the number of circulating white blood cells, levels of hormones and neurotransmitters, even the velocity of blood flow in the brain. Heart rate and blood pressure slowly increase over the day; the stress hormone Cortisol ebbs. With the onset of night come surges in the "darkness hormone" melatonin, a gradual fall in temperature, heart rate, and blood pressure, and a slow rise in Cortisol until its peak in early morning, before waking.

These circadian oscillations are hardly a trifling matter. If physicians don't take them into account, the measurements in a given individual of everything from blood pressure to heart rate, sperm count to allergic reactions, can be badly distorted. (Some scientists even argue that every clinical observation should be "time-stamped.") The rest of us can use our knowledge of these bodily ups and downs to make good personal choices. To avoid excessive bleeding, it's best to shave at 8
A.M
., when clot-forming blood platelets are more abundant and stickier than they are at other times of day (which also explains why heart attacks peak at this time). To escape those wincing twinges in the dentist's chair, time your visit for the afternoon, when the pain threshold in teeth is highest. If you want to minimize the damage to your body from alcohol, drink that beer or glass of wine between
5
and
6
P.M
., when the liver is generally most efficient at detoxifying booze. And if you want to set an athletic record, schedule your race for late afternoon or early evening.

So pervasive is the influence of our circadian cycles, says the chronobiologist Josephine Arendt, "that it would be reasonable to say that everything that happens in our bodies is rhythmic until proved otherwise."

So where is our little ticker? Pop into the bathroom for a minute and look in the mirror. If you could see into the dark interior of your skull, you might catch sight of a pair of tiny wing-shaped structures in the brain's hypothalamus, just behind and below your eyes, one in the right hemisphere and one in the left. These clusters of ten thousand neurons, collectively known as the suprachiasmatic nucleus (SCN), comprise the master clock in your brain. The SCN measures the passage of a twenty-four-hour day by producing and using special proteins in a circadian pattern. It controls and organizes the big rhythms of the body so that its sleep functions are optimal at night and its wakeful functions during the day. (When the SCN is destroyed in laboratory animals, their activities—running, eating, drinking, sleeping—follow no normal twenty-four-hour pattern but are randomly distributed across the day.)

With a full-length mirror and a little genetic engineering, you might also see the rest of your body ticking. We now know that the body has not one clock but billions: Circadian timekeepers are ticking away in virtually every bit of flesh, in kidney, liver, and heart, in blood and bone and eye. In one 2004 study, researchers inserted a gene for luciferase, a protein that gives fireflies their luminescent glow, to show in real time the circadian rhythms of cells in peripheral tissues. There they were, cells from all corners of the body, "blinking" in a circadian beat.

Though the master clock in the SCN oversees the body's cyclical rhythms, the genetic timepieces pocketed in the cells of outlying tissues and organs may follow their own daily routines, triggering peaks and troughs of activity at different times of day in their respective locations to ensure that a particular organ has what it needs when it needs it, and timing its activities according to its own priorities. The clocks in heart cells, for instance, set their own daily rhythms for blood pressure, and the clocks in liver cells, for digestion and for metabolism of toxins such as alcohol.

The body's peripheral timepieces have been likened to the instrument sections of an orchestra. The SCN is the conductor, coordinating the specific rhythms generated by these little clocks and synchronizing them according to light signals it receives from the outside world. But the peripheral clocks may step out and do their own thing—a phenomenon we may become aware of when we disrupt the symphony by traveling across time zones or working through the night.

What lies at the heart of every clock in the shop is a constellation of genes. Small variations in these clock genes may spell the difference between early birds happily up at dawn and those of more owlish bent, who struggle through the morning hours and hit their stride at midnight.

Louis Ptáĉek and his colleagues at the University of Utah were the first to reveal a direct genetic connection for extreme early birds. The team discovered that a large family of extreme larks living in Utah—patients with a disorder known as familial advanced sleep phase syndrome, who fell asleep as early as 7
P.M
. and awoke as early as 2
A.M
.—had a mutation in a central clock gene active in the SCN called
Per2.
Ptáček's team has since identified some sixty families with this gene. "These people were told they went to bed so early because they were depressed and antisocial," says Ptáček. Now it's clear they have a disorder related to their clock genes.

British scientists have shown that extreme larks and owls also tend to carry slightly different versions of the
Per3
clock gene. With remarkable consistency, very early risers had a longer variation of the gene than did late sleepers.

More moderate morning-evening preferences have also been correlated with such genetic variations. A team of scientists gave 410 subjects an owl/lark self-test to identify their preference for certain activities at certain times of day—time of rising, level of alertness on waking, favorite time for exercising and for doing mentally demanding tasks—to see where they might lie on the spectrum. The team also took blood samples from the subjects and compared the makeup of one of their clock genes. Those subjects with one variation of the gene showed a marked preference for eveningness, lagging as much as forty-five minutes behind larks in their preferred time for various activities.

As two prominent rhythm researchers have noted, "It seems that our parents—through their DNA—continue to influence our bedtimes."

Of course genes are not the whole story. Age matters, too. The transition from childhood to adolescence, especially, often sees dramatic shifts in avian tendencies. When Till Roenneberg studied the habits of twenty-five thousand people from the ages of eight to ninety, he found that children are typically early birds but start to become more owlish as they enter adolescence. The young child raring to go at 6
A.M
. morphs into an adolescent who would rather not rise until noon—as anyone knows who has tried to haul a teenager out of bed for an early school starting time. On free days and weekends, adolescents will delay their sleep phase by almost three hours. This pattern persists until about age nineteen and a half for women, almost twenty-one for men. In fact, says Roenneberg, the peak in owlishness can be used as a biological marker for the end of adolescence. After this, the avian pendulum often swings back, and we return to a more lark-like pattern.

Light also has bearing: Research by Roenneberg suggests that many of us are owlish because we don't get the natural light needed to advance our clocks. People who stay outdoors thirty hours or more a week tend to go to sleep and wake up two hours earlier than those who stay out only ten hours a week. But even spending just an hour or two in natural light early in the day can advance your clock by as much as forty-five minutes. So, if you want to tip your body toward more larkishness, consider walking to work.

Young or old, lark or owl, we are not at our best on awakening. I recently took part in a psychological study that asked me to monitor my alertness over the course of a day. I carried a Palm Pilot at all times, and when it beeped, I responded to several questions, then took a quick test to measure my reaction time.

Early mornings were an embarrassment.

Even as a confirmed lark, I know I need time to sweep away the cobwebs of sleep inertia and meet the day in full alertness—time and a drug, specifically the potent stuff found in a mug of strong coffee.

I'm hopelessly addicted. Once, on a trip to a remote corner of northeastern China, I spent the night in an old army barracks that featured broken windows, a hole-in-the-floor toilet, and a mattress riddled with cigarette burns. Knowing that coffee would be scarce, I had brought with me ground beans and a French press for making my own brew. But boiled water was not to be had. I confess that for my morning rush I resorted to chewing the dry grounds.

The rich aroma, the rattling kettle: Just the ritual itself promises clarity.

Bach loved coffee. So did Balzac, Kant, Rousseau, Voltaire, who is said to have consumed dozens of cups a day—and my mother, who drank a relatively modest six. Two hundred years ago, Samuel Hahnemann noted that for coffee drinkers "sleepiness vanishes, and an artificial sprightliness, a wakefulness wrested from Nature takes its place." Today coffee beans are the most widely traded commodity after oil, and caffeine is the world's most commonly used psychostimulant drug. More than 80 percent of people consume it in one form or another, in coffee, tea, maté, cacao, kola. Members of the Achuar Jivaro tribe of the Amazonian regions of Ecuador and Peru wake up each morning by drinking an herbal tea made from the leaves of a South American holly,
Ilex guayusa,
which contains caffeine equal to about five cups of coffee. So strong is the concoction that men usually vomit up most of it to avoid the symptoms of overdose: headache, sweating, jitters.

To banish my own morning stupor, I depend on the buzz of the 300 to 400 milligrams of caffeine in two mugs of strong coffee, which I down at one sitting. New research suggests that taking your caffeine this way—in one big dose, Achuar Jivaro style—does not give you the most bang from your beaker. Charles Czeisler and his team at Harvard found that a single helping of caffeine may cause a quick peak in alertness, but it rapidly falls off. The most effective way to combat fatigue, improve cognitive function, and avoid the jitters is to take your coffee in smaller doses, two ounces every hour or so.

Just why caffeine has such a potent effect on the body has only lately come into focus. From the bloodstream, the chemical diffuses throughout the body's tissues and fluids, not stopping to collect in any particular organ but circulating evenly in blood—and in amniotic fluid and fetal tissue. It raises blood pressure slightly, dilates the bronchi of the lungs, and allows the body quicker access to fuels in the blood. In your kidneys, it increases the flow of urine; in your colon, it acts as a laxative. It even boosts metabolic rate a little, which slightly accelerates the burning of fat. Within fifteen to twenty minutes, 90 percent of the caffeine in your cup has left your stomach and intestines and begun to affect your brain.