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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.

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