date 544 BC stamped on the other. Ernesto examines the coin—but instead of buying it, he calls the police. Why?
This is what social scientists call an “insight problem.” Reasoning in a methodical, algorithmic way won’t yield a correct answer. With insight problems, people typically begin with that systematic, step-by-step approach. But they eventually hit a wall. Some throw up their hands and quit, convinced they can neither scale the wall nor bust through it. But others, stymied and frustrated, eventually experience what’s called a “flash of illuminance”—aha!—that helps them see the facts in a fresh light. They recategorize the problem and quickly discover the solution.
(Still baffled by the coin puzzle? The answer will make you slap your head. The date on the coin is 544 BC, or 544 years before Christ. That designation couldn’t have been used then because Christ hadn’t been born—and, of course, nobody knew that he would be born half a millennium later. The coin is obviously a fraud.)
Two American psychologists, Mareike Wieth and Rose Zacks, presented this and other insight problems to a group of people who said they did their best thinking in the morning. The researchers tested half the group between 8:30 a.m. and 9:30 a.m. and the other half between 4:30 p.m. and 5:30 p.m. These morning thinkers were more likely to figure out the coin problem . . . in the afternoon. “Participants who solved insight problems during their non-optimal time of day . . . were more successful than participants at their optimal time of day,” Wieth and Zacks found.21
What’s going on?
The answer goes back to those sentries guarding our cognitive castle. For most of us, mornings are when those guards are on alert, ready to repel any invaders. Such vigilance—often called “inhibitory control”—helps our brains to solve analytic problems by keeping out distractions.22 But insight problems are different. They require less vigilance and fewer inhibitions. That “flash of illuminance” is more likely to occur when the guards are gone. At those looser moments, a few distractions can help us spot connections we might have missed when our filters were tighter. For analytic problems, lack of inhibitory control is a bug. For insight problems, it’s a feature.
Some have called this phenomenon the “inspiration paradox”— the idea that “innovation and creativity are greatest when we are not at our best, at least with respect to our circadian rhythms.”23 And just as the studies of school performance in Denmark and Los Angeles suggest that students would fare better taking analytic subjects such as math in the morning, Wieth and Zacks say their work “suggests that students designing their class schedules might perform best in classes such as art and creative writing during their non-optimal compared to optimal time of day.”24
In short, our moods and performance oscillate during the day. For most of us, mood follows a common pattern: a peak, a trough, and a rebound. And that helps shape a dual pattern of performance. In the mornings, during the peak, most of us excel at Linda problems— analytic work that requires sharpness, vigilance, and focus. Later in the day, during the recovery, most of us do better on coin problems—insight work that requires less inhibition and resolve. (Midday troughs are good for very little, as I’ll explain in the next chapter.) We are like mobile versions of de Mairan’s plant. Our capacities open and close according to a clock we don’t control.
But you might have detected a slight hedge in my conclusion. Notice I said “most of us.” There is an exception to the broad pattern, especially in performance, and it’s an important one.
Imagine yourself standing alongside three people you know. One of you four is probably a different kind of organism with a different kind of clock.
LARKS, OWLS, AND THIRD BIRDS
In the hours before dawn one day in 1879, Thomas Alva Edison sat in his laboratory in Menlo Park, New Jersey, pondering a problem. He had figured out the basic principles of an electric lightbulb, but he still hadn’t found a substance that worked as a low-cost, long-lasting filament. Alone in the lab (his more sensible colleagues were home asleep), he absentmindedly picked up a pinch of a sooty, carbon-based substance known as lampblack that had been left out for another experiment, and he began rolling it between his thumb and forefinger—the nineteenth-century equivalent of squeezing a stress ball or trying to one-hop paper clips into a bowl.
Then Edison had—sorry to do this, folks—a lightbulb moment.
The thin thread of carbon that was emerging from his mindless finger rolling might work as a filament. He tested it. It burned bright and long, solving the problem. And now I’m writing this sentence, and perhaps you’re reading it, in a room that might be dark but for the illumination of Edison’s invention.
Thomas Edison was a night owl who enabled other night owls. “He was more likely to be found hard at it in his laboratory at midnight than at midday,” one early biographer wrote.25
Human beings don’t all experience a day in precisely the same way. Each of us has a “chronotype”—a personal pattern of circadian rhythms that influences our physiology and psychology. The Edisons among us are late chronotypes. They wake long after sunrise, detest mornings, and don’t begin peaking until late afternoon or early evening. Others of us are early chronotypes. They rise easily and feel energized during the day but wear out by evening. Some of us are owls; others of us are larks.
You might have heard the larks and owls terminology before. It offers a convenient shorthand for describing chronotypes, two simple avian categories into which we can group the personalities and proclivities of our featherless species. But the reality of chronotypes, as is often the case with reality, is more nuanced.
The first systematic effort to measure differences in humans’ internal clocks came in 1976 when two scientists, one Swedish, the other British, published a nineteen-question chronotype assessment. Several years later, two chronobiologists, American Martha Merrow and German Till Roenneberg, developed what became an even more widely used assessment, the Munich Chronotype Questionnaire (MCTQ), which distinguishes between people’s sleep patterns on “work days” (when we usually must be awake by a certain hour) and “free days” (when we can awaken when we choose). People respond to questions and then receive a numerical score. For example, when I took the MCTQ, I landed in the most common category—a “slightly early type.”
However, Roenneberg, the world’s best-known chronobiologist, has offered an even easier way to determine one’s chronotype. In fact, you can do it right now.
Please think about your behavior during “free days”—days when you’re not required to awaken at a specific time. Now answer these three questions:
1. What time do you usually go to sleep?
2. What time do you usually wake up?
3. What is the middle of those two times—that is, what is your midpoint of sleep? (For instance, if you typically fall asleep around 11:30 p.m. and wake up at 7:30 a.m., your midpoint is 3:30 a.m.)
Now find your position on the following chart, which I’ve repurposed from Roenneberg’s research.
Chances are, you were neither a complete lark nor an utter owl, but somewhere in the middle—what I call a “third bird.”* Roenneberg and others have found that “[s]leep and wake times show a near-Gaussian (normal) distribution in a given population.”26 That is, if you plot people’s chronotypes on a graph, the result looks like a bell curve. The one difference, as you can see from the chart, is that extreme owls outnumber extreme larks; owls have, statistically if not physiologically,
