If you spend your workday at a desk, you know that familiar and dreaded feeling: the midafternoon slump.

E-mails take a bit longer to compose, documents take longer to read. It’s expected, given that you spend most of your time in a chair.

To combat your mental fatigue, you reach for artificial stimulation in the form of caffeine, sugar or energy drinks. But all you need to sharpen your mental acuity is something you already own: your legs.

This should come as little surprise. Mental fatigue sets in after you’ve been sitting for an extended period of time, and walking around is an easy way to temporarily increase your alertness.

But a few types of extended physical activity can have measurable impact on your mental acuity.

According to recent research, a single workout can immediately boost higher-order thinking skills, making you more productive and efficient as you slog through your workday.

When you exercise your legs, you also exercise your brain; this means that a lunchtime workout can improve your cognitive performance, thanks to blood flow and brain food.

Brain-derived neurotrophic factor, or BDNF, is a protein that facilitates the growth of neurons and nourishes existing ones.

It improves executive function, a type of higher-order thinking that allows people to formulate arguments, develop strategies, creatively solve problems and synthesize information.

BDNF sits idly at the synapses of your brain neurons and crosses the synapses only with the increased blood flow that comes with exercise.

Writers have long used exercise to unleash their creative powers. William Wordsworth composed his poems while walking.

According to Adam Sisman, author of “The Friendship: Wordsworth &Coleridge,” Wordsworth composed the 159-line “Tintern Abbey” in his head while on one of his walks: “The whole poem was carried in his mind; not a word of it was written down before they reached Bristol, and not a line altered afterwards.”

Novelist Susan Henderson often goes hiking for two or three hours. “For my first draft, I’ll go into the woods,” she said. “I’ll go with a specific question or dilemma, and talk it out into the voice recorder on my phone.”

Craig Finn of the band The Hold Steady uses running as a way to gather song ideas: “Long runs are a very meditative time. My mind gets to a crazy, unique place once I get above 10 miles. It’s a time for some very clear thinking. I don’t know that it’s conscious, but I always feel inspired to write after I run.”

Fortunately, you don’t have to run 10 miles to boost your executive function. Several studies have shown that a short aerobic workout gives your brain an immediate boost. According to Charles Hillman, professor of kinesiology and community health at the University of Illinois, as little as 20 minutes of aerobic exercise at 60 to 70 percent of your maximum heart rate is enough.

Your choice of exercise also makes a difference. A calm mind is key. The less you pay attention to external stimuli — like a book or your environment — the greater the benefit later, because any activity that requires extended concentration involves the same higher-order thinking skills you need after the run.

In other words, the more rested your mind during exercise, the better your post-workout problem-solving skills.