Consider this puzzle: An antiquities dealer is deciding whether to purchase a coin. The coin is made of bronze, with an emperor’s head engraved on one side and the date 544 B.C. stamped on the other. The dealer quickly determines the coin is fake. How?

Whether (or how quickly) you come up with the right solution might very well depend on the time of day you’re reading this, says Cynthia May, PhD, a professor of psychology at the College of Charleston in South Carolina.

The answer: Coins were never stamped “B.C.,” because the designation didn’t exist at the time. In solving the riddle, people are often carried away by their own logic, May explains. “They’re thinking, ‘When was bronze first used?’ and ‘Were there emperors during that time period?’ when in fact they have to let go of their first interpretation in order to solve the problem.”

Turns out, the best time of day to solve such “insight” problems, or to undertake a creative challenge, is not the same time of day you’d want to take a math test or solve a problem that requires careful analysis. Researchers are turning up all kinds of evidence that one’s cognitive performance fluctuates in predictable patterns throughout the course of a day. “The time at which you do things matters,” May says.

Afternoon slumps

Researchers from a variety of fields have noted some of the ways that timing makes a difference, in matters from medicine to business. Madhusudhan Sanaka, MD, at the Cleveland Clinic, and colleagues studied colonoscopy data from more than 3,600 people and found that physicians identified significantly more abnormalities during morning colonoscopies than during those performed in the afternoon (The American Journal of Gastroenterology, Vol. 104, No. 7, 2009).

Moods, too, rise and fall predictably during the day. Scott Golder, PhD, and Michael Macy, PhD, at Cornell University, studied language from millions of public Twitter posts from around the globe. They found the average Twitter user had a happiness spike around breakfast, hit a grumpy slump in late afternoon and perked up again after dinner (Science, Vol. 333, No. 6051, 2011). “We found an incredibly robust pattern, across diverse cultures all over the world,” Macy says.

The time-sensitive nature of moods can have surprising ripple effects as well. Jing Chen, PhD, at the University of Buffalo School of Management, and colleagues analyzed quarterly earnings conference calls and found that financial executives and analysts were upbeat in the morning and became more negative as the day wore on. Those mood changes led the analysts to make more errors related to stock pricing in the afternoon (Management Science, online first publication, 2018).

Given these findings, should you schedule important tasks for the morning and give afternoons over to an extended siesta? It’s not that simple, other research finds. People’s cognitive abilities fluctuate throughout the day in accordance with their personal circadian patterns, or chronotypes.

We all fall into one of five different chronotypes, defined by the window of time we feel most alert and energetic, May explains: strong morning types, moderate morning types, strong evening types, moderate evening types and those who are neutral, who peak midday. Whether you bounce out of bed before sunrise with the larks, prefer the company of owls and bats or hover somewhere in between, most people know intuitively which category they belong to. If you’re not sure, a systematic assessment can help you figure it out. Researchers use several surveys to measure chronotype, including the Morningness-Eveningness Questionnaire (MEQ), the Composite Scale of Morningness (CSM) and the Munich Chronotype Questionnaire (MCTQ).

Our daily patterns are more than mere preferences, though, says Dorothee Fischer, PhD, a research fellow at Harvard Medical School/Brigham and Women’s Hospital who studies circadian rhythms. “Chronotype isn’t a personality trait, but a biological characteristic,” she says.