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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.

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


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