What should I do? is a question that receives a lot of attention. Frequently we overlook another important question—When is the best time of day to do that? But a growing body of ­research suggests that picking the wrong time of day to perform a challenging task can dramatically reduce your odds of success—and the wrong time of day to do certain things is not the same for everyone.

Here’s how to use this knowledge to avoid problems and increase your success…

THE AFTERNOON PLUNGE

It’s no secret that fatigue can sap our skills late at night, but it turns out that most people’s batteries are nearly as drained in the mid-to-late afternoon—approximately 2 pm to 4 pm. This is more than just a little post-lunch sluggishness. Students who take standardized tests in the afternoon, rather than in the morning, experience score declines as severe as if they had missed about a half month of school. Medical professionals who examine colonoscopy scans in the afternoon spot only half as many polyps as in the morning. Fatigue-related traffic accidents peak twice each day—in the middle of the night…and between 2 pm and 4 pm.

Our moods follow a similar pattern—a study published in Emotion found that feelings of warmth toward others and overall happiness climb in the morning, then crash in early afternoon before recovering in late afternoon and evening.

What to do: Don’t think of the midafternoon slump as a minor obstacle or a joke—it is no joking matter. Unless you are a night owl (more on that later), to the extent you can, avoid making important decisions or performing mentally challenging tasks between 2 pm and 4 pm. Tackle these tasks before or after that time window.

MORNING OR EVENING?

If you are going to move some of your more challenging life activities away from 2 pm to 4 pm, when should you do them? Morning typically is best for thought that requires vigilance and attention to detail, while late afternoon and early evening seem to be particularly good times to work on creative ideas. This might be because the mind is sharp enough to think clearly in the late afternoon/early evening, but it’s still unfocused enough to drift, allowing it to form creative connections.

Consider these factors not only when scheduling what you do but also what’s done for you. Examples: Schedule medical procedures and work by skilled professionals such as auto mechanics and plumbers for the morning. Otherwise, you might not get these pros when they are at their sharpest.

REDUCING THE PLUNGE

There is a wonderful way to minimize the mental drag of your afternoon plunge—take a nap. Numerous studies have found that napping improves problem-solving skills, information retention, alertness and reaction times. Yet most adults rarely or never nap—some because their work schedules do not allow it, but others because they don’t think they have time for naps…or because they tend to wake from naps feeling foggier than ever. But these people generally are napping wrong. The key to successful napping is to nap for only 10 to 20 minutes, no longer.

What to do: Just before a nap, drink a cup of coffee (or something else containing caffeine) and set an alarm to sound in 25 minutes. That gives you five to 15 minutes to fall asleep, followed by the appropriate 10 to 20 minutes of sleep. It takes caffeine 25 minutes to reach the bloodstream, so drinking coffee right before a short nap means that the caffeine kicks in just when you need a boost to shake off any grogginess from the nap. A Mayo Clinic study found that for most people, napping between 2 pm and 3 pm provides the biggest boost.

If napping is not an option, take a few minutes for a short, invigorating walk, ideally in nature…a brief social visit or phone call with someone whose company you enjoy…listening to a ­humorous podcast…or meditating.

THE NIGHT OWL EXCEPTION

Approximately one person in five is a “night owl” who is mentally sharpest in the late afternoon or early evening.

If you’re not sure whether you’re a night owl, note when you tend to fall asleep and when you tend to wake on nights when you have the freedom to go to sleep whenever you like because you have nothing on your schedule the following day. If the midpoint of these fall-asleep and wake-up times is 6 am or later, you’re a night owl. Example: If you find yourself staying up until 2 am and waking up at 10 am during vacations, you’re a night owl—6 am is the midpoint between 2 am and 10 am.

Also be aware that night-owlism can come and go during a lifetime. People are most likely to be night owls in their teens and early 20s…and least likely to be night owls before age 12 and after age 60. But anyone of any age can be a night owl.

The slump that hits most people in the early afternoon is more likely to affect a night owl later in the day, starting approximately six to seven hours after rising—but only if that night owl has the freedom to wake late and go to bed late. The daily ups and downs of a night owl who is forced by a job or other responsibilities to wake early are more difficult to predict.

What to do: There is no “cure” for being a night owl—but there is nothing inherently wrong with it, either. Still, night owls often struggle to fit themselves into a world that tends to be scheduled for early risers. Though there isn’t much research on night owls and napping, if your schedule permits, you can try taking a nap (see instructions above) in the late afternoon or early evening. Finding a job that allows flexible working hours can work well for night owls. If that is not possible, night owls should at least try to avoid doing their most demanding work or making key decisions in the few hours starting about six or seven hours after they wake up.

TIMING LONG-TERM PROJECTS SO THEY SUCCEED

Besides their daily ebbs and flows of energy and focus, people also tend to have longer-term ebbs and flows in their ability to accomplish ongoing projects. Here’s how to avoid that phenomenon and have more success with your long-term projects…

Begin big projects (or implement major changes) on a “fresh start date.” This includes the most obvious date—January 1—but also could be your birthday, the first day of a month or even just a Monday. It is psychologically easier to start new things on these days because your mind can imagine a new version of you beginning on this day, leaving behind the old, “flawed” version of you.

At the midpoint of a project, it’s common to experience a lull because the excitement of the start has passed and the end point still seems far away. It might help to rally yourself with I really need to get going self-talk (or rally your team in a group effort)—but even better is to say, “I/we really are in trouble.” Why: Researchers have found that a midpoint slump can be converted into a midpoint jump by taking advantage of the “uh-oh” effect—that is, by scaring yourself with the fact that the halfway point has arrived and that there still is a lot left to do.

At the end of a project, find a way to celebrate its conclusion. The end of an effort is what you and others are most likely to remember about it, so if you just let things drift to an end, you’re wasting an opportunity. Instead, review what you achieved…and reward yourself (and/or your team) with a bonus or gift—a token gesture is fine. This greatly improves the odds that you will remember the effort fondly, which will make it easier to tackle other big projects in the future.