Caffeine No Substitute For A Nap To Enhance Memory
Hoping to improve your tennis serve? It's probably better to catch a few winks than load up on java after a lesson, results of a NIMH-supported study suggest. Caffeine impaired such motor learning and verbal memory, while an afternoon nap benefited all three types of learning tested by Sara Mednick, Ph.D., and colleagues at the University of California, San Diego.
Ninety percent of Americans use caffeine daily, some substituting it for sleep. While the stimulant enhances alertness and concentration, it's been unclear whether it also helps learning and memory. By contrast, daytime naps, like nighttime sleep, benefit both alertness and memory, Mednick and colleagues have shown in a series of studies.
In this first head-to-head day-time comparison, 61 participants trained in the morning on verbal memory, motor, and perceptual learning tasks. After lunch, one group napped (60-90 min), while two other groups listened to a book on tape and received a pill containing either the caffeine equivalent of a little less than a Tall Starbucks brewed coffee (200mg) or a placebo. Later in the afternoon, the three groups were tested to see how well they had learned the tasks.
Findings of This Study
The nap group performed significantly better on a finger tapping motor task and in recalling words, than the caffeine group. The nap group also trumped the other groups on a texture discrimination task of perceptual learning. The placebo group performed better than the caffeine group on all three tasks. Curiously, just thinking that the pill might contain caffeine — the placebo effect — helped as much as a nap on the motor task.
Evidence suggests that caffeine interferes with tasks that require processing explicit, as opposed to implicit, information - like recalling a specific word, versus remembering how to type or ride a bike. Studies show that consolidation of such explicit verbal memory during sleep depends on lowered levels of the chemical messenger acetylcholine in the brain's memory hub. Yet, by blocking activity of a natural sedative chemical, caffeine boosts acetylcholine in this hub.
"This increase in acetylcholine by caffeine may impair the consolidation process by blocking replay of new memories," proposes Mednick. "Consistent with this, we found that the greater the explicit component of each task, the worse the caffeine group performed."
"Such an impairment of performance runs counter to society's assumption that caffeine typically benefits cognitive performance," she notes. "Apparent improvements with caffeine might actually reflect a relief from withdrawal symptoms. Just as no medicinal alternative to a good night's rest has been discovered, so too caffeine, the most common pharmacological intervention for sleepiness, may not be an adequate substitute for the memory enhancements of daytime sleep, either."
Mednick and colleagues are using new pharmacological agents found to selectively enhance particular stages of nighttime sleep to see if they can enhance memory consolidation during daytime naps. Brain imaging will pinpoint effects on neural circuits. These studies of pharmacologically enhanced naps could lead to improved treatments for memory impairment in mental disorders, based on manipulations of sleep, say the researchers.