It’s 1:30 p.m., post-lunch, and your eyelids are feeling heavy. Your boss called you this morning and needed you to do something … but what was it? Your brain may be asking for a rest, but you’ll likely reach for stimulation instead — about 90 percent of Americans use caffeine every day.
Though it should make you feel more awake, caffeine probably does nothing to help you recall what was said this morning. A new study, just published in the journal Behavioural Brain Research, shows that an hour of shut-eye is a far more effective restorer of memory than a cup of coffee.
The study’s main author, Sara Mednick, is a professor of psychiatry at the University of California, San Diego. In 2002, she tested visual perception in a group of college students, finding that even in well-rested individuals the ability to discern shapes declines steadily throughout the day. A 30-minute afternoon nap halts further deterioration, however, and a 60-minute nap actually restores people to their morning capacity.
Such findings led her to become an ardent nap advocate: She wrote a general-interest book called Take a Nap! Change Your Life and moonlights as a corporate nap consultant.
Of course, modern worker bees rarely have the luxury of an hourlong midday doze. Various studies have verified that in the right dose — which varies from person to person — caffeine enhances alertness and concentration. But its effects, if any, on higher cognitive functions like learning and memory are less clear.
For Mednick’s experiment, her team recruited a group of undergrads and divided them into three groups — caffeine, nap and placebo. Each group would be tested on three different facets of memory — verbal, motor and perceptual. After a full night’s rest, testing began: For the verbal task, subjects listened to a list of unrelated words. The motor task measured the accuracy of their fingers tapping a specific sequence on a keyboard. For the perceptual task, they were judged by the speed at which they could identify the orientation of bars on a screen.
At midday, one group was treated to a 60- to 90-minute nap, while the others took a placebo or caffeine pill — at 200 milligrams, about the same as a regular Dunkin’ Donuts coffee. Seven hours after the first morning session, subjects were retested on what they had learned. For Mednick, the most striking result was in the verbal area — nappers significantly outperformed the placebo and caffeine groups in the number of words they could recall. For motor accuracy, nap and placebo beat caffeine; for perceptual quickness, nap and caffeine beat placebo.
Mednick believes her results reflect the fact that verbal, motor and perceptual memory each involves a different region of the brain. Recalling a list of words — or specific instructions from a boss or driving directions — engages the hippocampus, a region that recharges during the period of deep sleep called stage 4, or “slow wave sleep.” This kind of deliberate, explicit learning therefore benefits most from a generous midday nap.
Perceptual memory, on the other hand, is innate — through repeated exposure, the visual system learns to recognize and respond without conscious thought. Radiologists, for example, tap into their perceptual memory to identify tumors in an X-ray.
Motor memory lies somewhere in between — at first learning is deliberate but soon feels as intuitive as riding a bicycle. In this area, Mednick also noted an increased benefit from napping and evidence of overstimulation in the declining accuracy of the caffeinated finger-tappers.
Whether caffeine adversely affects any type of memory remains an open question —other researchers believe its effects are neutral except in sleep-deprived individuals, where it can be a boon.
Nonetheless, Mednick’s lesson is clear: For a midday memory boost, nap clearly trumps caffeine. “It’s an investment of time to take a 60-minute nap, though if you think of it, the time difference between going to Starbucks for a coffee may be only half an hour,” she said. “If there’s a chance of flagging, I’d rather take a nap.”
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