Stills from Christian Marclay's The Clock, on view this summer at Lincoln Center.Photo: Christian Marclay. Courtesy of Paula Cooper Gallery, New York and White Cube, London.

It was early on a summer evening in 1729, and the French astronomer Jean Jacques d’Ortous de Mairan was procrastinating. When I’m procrastinating I tend to stare off into space, but that would have been too much like work for an astronomer, so De Mairan stared at a houseplant. Specifically, the mimosa on his windowsill.

Everything is more interesting than your work when you’re procrastinating, including a potted plant, and so De Mairan found himself thinking about the mimosa. Its leaves were furled up for the evening. How did they know when to do that? De Mairan recalled the way plants track the sun (picture flowers gradually turning their faces from east to west), and concluded that the leaves’ behavior was triggered by the waning light.

Curious to test his hypothesis, he placed the mimosa in a dark cabinet, theorizing that the light-deprived leaves would stay closed. The next day, he found them unfurled. Surprised, he let the experiment run for a while. Every evening, when he peered into the cabinet, the leaves were closed; every morning, they were open. So much for his hypothesis: When it came to the timing of leaf behavior, the plant, not the sun, was running the show.

In his accidental break from astronomy, De Mairan provided the first published evidence that organisms possess internal clocks. He also launched an entirely new field—chronobiology, the study of those clocks and of how the cyclical processes of living beings relate to the cyclical processes of the cosmos. Today, chronobiology sheds light on everything from when we get hungry and tired to when we’re smartest, horniest, most dexterous, best able to tolerate alcohol, and least affected by pain. It includes subspecialties like chronoastrobiology (internal clocks help illuminate the origins of life on Earth) and chronopharmacology (there’s a reason you’re supposed to take your meds on time).

All this I learned from German scientist Till Roenneberg’s Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired. As science books go, Internal Time is no Botany of Desire, but nor is it Human Molecular Genetics, Fifth Edition. Reading it, I vacillated between wishing the topic had been taken up by one of the great science writers of our day and thinking, With content this interesting, who cares? Internal Time made me think deeply about what it means to be a time-bound organism: about the ways we live in time and the ways time lives in us. It is, in an unusually literal sense, a book about what makes us tick.

Modern human beings are not much like mimosas. It’s true that both have biological clocks, but only one of us has culture. And culture, delightful as it is, turns out to radically complicate—“fuck up” would not be an overstatement—our relationship to time.

Among species, we humans are to time what Polish villagers have long been to place: unhappy subjects of multiple competing regimes. The first regime is internal time: the schedule established by our bodies. The second is sun time: the schedule established by light and darkness. These two we share with houseplants and virtually every other living being. But we are also governed by a third regime: social time. That sounds benign enough, like afternoon tea with a friend. But don’t be fooled. Social time is the villain in this drama, out to turn you against health, happiness, nature, sanity, even your own inner self.

An “internal clock” is not a metaphor. Or rather, it is—you don’t have a tiny Timex in your cerebellum—but it’s also a real biological feature, a specialized bundle of cells that regulates our cyclical processes. These clocks are remarkably widespread. Single-cell creatures that lack even nuclei nonetheless have internal clocks; so do human beings with programmable cappuccino-makers. In plants, the clock can be located in leaves, stems, or roots. In slugs, it’s at the base of the eye. In many birds, it’s in the pineal gland, the structure near the center of the brain where Descartes thought future scientists would find the soul.

In mammals, the clock is located near the base of the brain, in a group of nerve cells known as the suprachiasmatic nucleus, or SCN. The SCN consists of only about 20,000 of the brain’s estimated 100 billion neurons; you could fit the entire thing on the tip of the second hand of an analog watch. Yet without it, you are profoundly screwed. If you replace the SCN of one hamster with that of another, the original hamster will begin sleeping, eating, and attending its manic hamster spin class on the schedule previously maintained by the other one. If you remove the SCN, the hamster’s behavior will lose all regularity. Similarly, people with brain lesions in the SCN region cannot maintain consistent sleep-wake patterns.

Assuming that you have a functioning SCN, you also have a chronotype—a genetically determined blueprint for sleepiness, hunger, hormone levels, body temperature, and so forth. Of these, Roenneberg focuses primarily on sleep, “the most conspicuous expression of the body clock in humans.”

That expression takes two forms: sleep timing (when you go to bed) and sleep duration (how much sleep you need). These variables are independent; you can be an early bird who needs ten hours of sleep, a night owl who needs six, or vice versa. You can also be neither. Sleep patterns form a bell curve, and the vast majority of people fall in the middle. What you cannot do—contrary to popular opinion—is change your clock through sheer force of will.

As a chronotypical outlier, I know this firsthand. Work-wise, I function best from around 10 p.m. to 4 a.m., a characteristic I share with roughly one percent of the population. That’s not an easy schedule to live with, so I once tried to train myself into a nine-to-five workday instead. Dismissing my conviction that I wrote better at night as so much Romantic preciousness, I diligently sat down to work each morning, spent eight hours watching the daylight fill and drain outside my window, then finally, well after dark, abruptly found myself able to write. After six months of this insanity—during which I more than doubled my workday without remotely upping my productivity—I gave up and went back to the other, better kind of craziness. The moral applies to every internal clock: Good luck trying to buck it.

Left to their own devices, internal clocks can get much stranger than mine. Roenneberg cites experiments in which subjects were confined to bunkers and deprived of all temporal cues. While most subjects maintained a day-night periodicity of roughly 24 hours (circa one day: hence, “circadian”), some people’s cycle doubled, to about 48 hours. Amazingly, they were oblivious to the change. They continued to eat three meals a “day,” and their sense of smaller time units doubled, too. Asked to estimate an hour, they estimated two instead.

Fortunately, internal clocks are seldom left to their own devices. Instead, they are “entrained,” as chronobiologists say, by sun time. Entraining is not a mystical Stonehenge-y sun-worshipping thing: It’s a precise biochemical process, conducted by a light receptor in your eye, melanopsin, neglected stepsister of the more famous rods and cones. Just as your biological clock runs free if you remove natural light, it runs free if you remove melanopsin: Barring medical intervention, some blind and almost all eyeless people are impervious to sun time and cannot be entrained.

Most of us are spared that fate—but, increasingly, we all live in bunkers. The average American spends more than 21 hours per day indoors, and when it comes to entraining a biological clock, indoor lighting cannot hold a candle, so to speak, to the sun. A well-lit indoor space maxes out at about 200 lux, a unit of light intensity. A rainy day measures around 10,000 lux. A sunny day can hit 150,000. Meanwhile, as we’re exposed to less natural light, we’re also exposed to more artificial light—i.e., less darkness. As a result, the contrast between day and night (in technical terms, the amplitude of the light-dark cycle) is much smaller than it was for our preindustrial ancestors. We live, Roenneberg writes, in “constant twilight.”

The impact on our sleep is significant. One researcher he cites found that nursing-home residents sleep poorly in part because they’re exposed to almost no natural light. Similarly, the average chronotype of people in cities is later than that of rural dwellers—and the larger the city, the later it gets.

Of course, urbanites also keep later hours thanks to the third temporal regime: social time. For most of history, social time was closely aligned with sun time, for the obvious reason that there are limited (albeit interesting) things people can do in the dark. Modernization changed all that. Today, almost one in five workers in the industrialized world does shift work. And countless people hold jobs located countries or continents away—from Filipinos in call centers to traders on their Bloomberg terminals, checking Greek stock prices at 2 a.m.

Perhaps the most explicit shift away from sun time came around the turn of the twentieth century with the gradual adoption of global time zones. Before then, all time was local, typically dictated by a church or town-hall clock. That system kept social time and sun time aligned—the clock read noon when the sun reached its zenith—but it became inconvenient in a nationalizing and then globalizing world. Time zones, by contrast, are convenient but contrived. If you are in, say, western Spain during daylight saving time, the clock will read midnight at what is, by sun time, 9:22 p.m.

Time zones were invented partly to simplify railroad schedules, but these days we associate them most with air travel—specifically, with jet lag, that highest manifestation of our increasingly unnatural relationship to time. Jet lag feels terrible, Roenneberg says, not just because you’re out of sync with the world but because you’re out of sync with yourself: New research suggests internal organs adjust to time changes at different rates. In effect, while your conscious mind is in Paris, MapQuesting its way toward salted-caramel ice cream on the Île Saint Louis, your SCN is in New York, and your liver is somewhere over the mid-Atlantic.

Ultimately, though, Roenneberg is more interested in what he calls “social jet lag”: the exhaustion produced by the gap between internal and social time. You can, should you choose, quantify your social jet lag. Simply calculate the difference between the midpoint of your average night’s sleep on a workday and a day off. Say on workdays you fall asleep at eleven and wake up at six: Your midpoint is 2:30 a.m. On weekends, you fall asleep at one and wake up at nine: Your midpoint is 4:30—and you’ve got two hours of social jet lag. You might as well fly from New York to Utah.

Social jet lag, unlike real jet lag, is chronic. Its chief symptom is sleep deprivation, and sleep deprivation is—surely I do not need to tell you this—ghastly. It leaves you with the equilibrium of a despot, the attention span of a toddler, and the working memory of a fire hydrant. It’s one of the few human conditions that can make the characteristics of the tomb—dark, quiet, horizontal—seem unbelievably desirable. Not for nothing are torturers so fond of it.

Physiologically, sleep deprivation is even more alarming. Even small amounts of it produce mood disruptions, especially in children. (Recent studies suggest that some kids diagnosed with ADHD might just need more sleep.) Roenneberg reserves special ire for school systems, which he says nurture a delusional “disco hypothesis” about adolescents: If they didn’t stay up late and party, they’d be perfectly functional in algebra class at 8 a.m. Baloney, Roenneberg retorts. Kids stay up and party because they have late chronotypes, not the other way around. When one researcher brought high-school students to her lab instead of to school in the morning, they instantly fell into REM sleep. The ability to do so is normally limited to narcoleptics.

Adults, too, rapidly lose their equilibrium in the face of even short-term sleep loss. Long-term, it’s associated with depression, diabetes, obesity, cardiovascular problems, and cancer. Your odds of being a smoker rise significantly for every hour of social jet lag you suffer. The World Health Organization recently classified “shift work that involves circadian disruption” as a potential carcinogen. The physiological, in other words, bears out the phenomenological. Sleep deprivation makes us sick, sad, and dumb.

“What is time?” Augustine asked in his Confessions. Beats him: “If no one asks me, I know. If I wish to explain it to one that asketh, I know not.” Fifteen hundred years later, we remain similarly befuddled. We think about time constantly: when our alarm goes off in the morning, when we glance at our wristwatch mid-run, when we watch our children grow shockingly tall. We also talk about time constantly. In a very different book on the subject, the excellent From Eternity to Here, the physicist Sean Carroll notes that time is the most-used noun in the English language.

And yet, despite all this, time is maddeningly difficult to define. Roenneberg, tellingly, doesn’t even try. One widely accepted definition—endorsed by, among others, Einstein—is: “Time is what clocks measure.” That definition seems tautological, which is sort of the point. As Einstein established, time is relative, affected by whatever does the measuring.

One of those whatevers is us. Time is what we measure, not just with our external Einsteinian clocks but with our internal Roennbergian ones: heart rate, hunger, breath, sleep. Like almost every other species, we humans are a kind of mobile timepiece. Unlike other species, we’ve overrun our niche in the temporal ecosystem, just as we have in the physical one. We move as freely from time to time as we do from place to place—working nights, jetting three hours into the past for a long weekend. That remarkable temporal suppleness, like our adaptiveness more generally, both rewards and imperils us. We live in all time but, unlike De Mairan’s mimosa, we live uneasily in it, struggling to balance our inner self with the demands of nature and each other.

And we live uneasily in time in another way, too. Time is what all creatures measure, but humans are the creatures who measure time. That is a remarkable but not a comfortable ability. If human culture is delightful but disrupts our sleep, the same could be said of human consciousness. It’s wonderful, thank heavens for it—and yet we are the only species kept awake at night by the thought that time is passing, that its quantity, for us, is finite. This is the fundamental pathos of being, in effect, a conscious Swatch. Our internal clocks do what we cannot: keep time.

Confessions of a Literary Night Owl

Internal Time: Chronotypes, Social Jet Lag, and Why You’re So Tired
By Till Roenneberg.
Harvard University Press. 288 pages. $26.95.