When: The Scientific Secrets of Perfect Timing(6)

When researchers have posed the Linda problem at different times of day—for instance, at 9 a.m. and 8 p.m. in one well-known experiment—timing often predicted whether participants arrived at the correct answer or slipped on a cognitive banana peel. People were much more likely to get it right earlier in the day than later. There was one intriguing and important exception to the findings (which I’ll discuss soon). But as with executives on earnings calls, performance was generally strong in the beginning of the day, then worsened as the hours ticked by.13

The same pattern held for stereotypes. Researchers asked other participants to assess the guilt of a fictitious criminal defendant. All the “jurors” read the same set of facts. But for half of them, the defendant’s name was Robert Garner, and for the other half, it was Roberto Garcia. When people made their decisions in the morning, there was no difference in guilty verdicts between the two defendants. However, when they rendered their verdicts later in the day, they were much more likely to believe that Garcia was guilty and Garner was innocent. For this group of participants, mental keenness, as shown by rationally evaluating evidence, was greater early in the day. And mental squishiness, as evidenced by resorting to stereotypes, increased as the day wore on.14

Scientists began measuring the effect of time of day on brainpower more than a century ago, when pioneering German psychologist Hermann Ebbinghaus conducted experiments showing that people learned and remembered strings of nonsense syllables more effectively in the morning than at night. Since then, researchers have continued that investigation for a range of mental pursuits—and they’ve drawn three key conclusions.

First, our cognitive abilities do not remain static over the course of a day. During the sixteen or so hours we’re awake, they change—often in a regular, foreseeable manner. We are smarter, faster, dimmer, slower, more creative, and less creative in some parts of the day than others.

Second, these daily fluctuations are more extreme than we realize. “[T]he performance change between the daily high point and the daily low point can be equivalent to the effect on performance of drinking the legal limit of alcohol,” according to Russell Foster, a neuroscientist and chronobiologist at the University of Oxford.15 Other research has shown that time-of-day effects can explain 20 percent of the variance in human performance on cognitive undertakings.16

Third, how we do depends on what we’re doing. “Perhaps the main conclusion to be drawn from studies on the effects of time of day on performance,” says British psychologist Simon Folkard, “is that the best time to perform a particular task depends on the nature of that task.”

The Linda problem is an analytic task. It’s tricky, to be sure. But it doesn’t require any special creativity or acumen. It has a single correct answer—and you can reach it via logic. Ample evidence has shown that adults perform best on this sort of thinking during the mornings. When we wake up, our body temperature slowly rises. That rising temperature gradually boosts our energy level and alertness—and that, in turn, enhances our executive functioning, our ability to concentrate, and our powers of deduction. For most of us, those sharp-minded analytic capacities peak in the late morning or around noon.17

One reason is that early in the day our minds are more vigilant. In the Linda problem, the politically tinged material about Linda’s college experiences is a distraction. It has no relevance in resolving the question itself. When our minds are in vigilant mode, as they tend to be in the mornings, we can keep such distractions outside our cerebral gates.

But vigilance has its limits. After standing watch hour after hour without a break, our mental guards grow tired. They sneak out back for a smoke or a pee break. And when they’re gone, interlopers—sloppy logic, dangerous stereotypes, irrelevant information—slip by. Alertness and energy levels, which climb in the morning and reach their apex around noon, tend to plummet during the afternoons.18 And with that drop comes a corresponding fall in our ability to remain focused and constrain our inhibitions. Our powers of analysis, like leaves on certain plants, close up.

The effects can be significant but are often beneath our comprehension. For instance, students in Denmark, like students everywhere, endure a battery of yearly standardized tests to measure what they’re learning and how schools are performing. Danish children take these tests on computers. But because every school has fewer personal computers than students, pupils can’t all take the test at the same time. Consequently, the timing of the test depends on the vagaries of class schedules and the availability of desktop machines. Some students take these tests in the morning, others later in the day.

When Harvard’s Francesca Gino and two Danish researchers looked at four years of test results for two million Danish schoolchildren and matched the scores to the time of day the students took the test, they found an interesting, if disturbing, correlation. Students scored higher in the mornings than in the afternoons. Indeed, for every hour later in the day the tests were administered, scores fell a little more. The effects of later-in-the-day testing were similar to having parents with slightly lower incomes or less education—or missing two weeks of a school year.19 Timing wasn’t everything. But it was a big thing.

The same appears to be true in the United States. Nolan Pope, an economist at the University of Chicago, looked at standardized test scores and classroom grades for nearly two million students in Los Angeles. Regardless of what time school actually started, “having math in the first two periods of the school day instead of the last two periods increases the math GPA of students” as well as their scores on California’s statewide tests. While Pope says it isn’t clear exactly why this is happening, “the results tend to show that students are more productive earlier in the school day, especially in math” and that schools could boost learning “with a simple rearrangement of when tasks are performed.”20