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.