Many people knock back a cup of coffee every morning with the goal of doing better on a test or jumping into a backlog of work … or just to feel remotely human. Caffeine is the most widely used stimulant in North America—some 90 percent of the adult population consumes it for its mentally arousing effects. But a recent study published in Frontiers in Psychology has found that the time of day and your age may influence coffee's influence on your memory.

Memory, of course, is a key component of learning, retention, and performance in a number of areas of our lives. Explicit memory, lead author Stephanie Sherman, a postdoctoral fellow in the psychology department at Boston College, tells mental_floss, "is just conscious recall of info." Implicit memory, on the other hand, which researchers call "priming," is the unconscious recall of previously learned information. More specifically, if something you’ve recently encountered happens to be in the forefront of your mind, "you’re more likely to recall it without being conscious you recalled it because you previously saw it," she explains.

Sherman became intrigued by a study her professor Lee Ryan conducted that explored the relationship between caffeine and implicit and explicit memory in older adults. Ryan’s study showed that caffeine enhanced memory performance in older adults in their non-optimal time of day: the afternoon. But caffeine had no effect on their memory in the morning, Sherman says. She wanted to find out if this would hold true in young adults, whose circadian rhythms are different, making afternoon their optimal time of day for physiological arousal—essentially, in how awake they feel—and morning their non-optimal time.

To test this hypothesis, she and her colleagues designed a double-blind experiment with college students aged 18 to 21. The first group of students came into the lab between 6:00 and 7:00 a.m. Half of these participants received a cup of caffeinated coffee, and the others got decaffeinated; regardless of which cup they received, they were told their coffee was caffeinated. That procedure was repeated with a second group of 40 participants in the afternoon between 2:00 and 4:00 p.m. Sherman’s team theorized that caffeine would have memory-boosting effects for young adults in their non-optimal time of day—early morning—but to determine the specificity of caffeine’s effects, they also investigated the effects on memory performance of two types of exercise in the early morning—vigorous aerobic exercise and gentle stretching.

"We thought another way of increasing physiological arousal would be exercise. If just being more awake increases your memory performance, than exercise would have the same effect [as caffeine]," Sherman reports.

On arrival, participants reported how awake they felt on a scale from one (not awake) to five (wide awake). After drinking one cup of coffee, they began two memory tasks. The first was a word stem completion task, in which they looked at words and were asked to tell the researchers how pleasant the word was on a scale from one to five. This was what Sherman calls incidental encoding: "They’re not told why they’re viewing it except to rate the pleasantness of these words." The goal was to plant words in the participants’ minds for potential recall in the next test.

In the next test, for implicit memory, participants engaged in a word-stem completion test. Each participant would see a word root, such as bas, and have to complete the word. Twenty-four of the stems could be completed with words they had seen during the study phase. "So if they see bas, for example, and in the study phase they were primed and had seen the word baseball, they might say 'baseball,'" Sherman explains. "Otherwise, they might say 'basement,' 'base,' or any other world that starts with bas. That’s the test of implicit memory."

To test explicit memory, the scientists performed a cued recall test consisting of word stems which corresponded to words from the study list. If participants saw bas, for example, and remembered that during the study phase they’d seen baseball, they should have been able to recall baseball with ease.

To compare the effects of caffeine to exercise, participants in the second experiment engaged in approximately 15 minutes of cardiovascular exercise or gentle stretching during the early morning. The authors write, "If caffeine benefits explicit memory by increasing general physiological arousal, we would expect to see the same boost in memory performance after morning exercise."

The results should be encouraging to coffee addicts: Researchers found young adults who drank caffeinated coffee in the morning were 30 percent better at the cued recall test than their decaf counterparts were. They also reported being significantly more awake by the end of the experiment than those who drank decaf. However, the two groups showed no difference in the implicit memory test, and for those who performed their tests in the afternoon, caffeine did not make a significant improvement in either test.

Sherman says that several of their results were surprising: First, that caffeine did not have much effect on participants in their optimal time of day (afternoon). Sherman theorizes, "The idea is that if people are already at their optimal, some caffeine is not going to further increase performance. Caffeine only helped when you’re at your low point in the day of physiological arousal and performance." Also unexpected: Exercise did not improve memory performance either.

Sherman makes clear that this isn't a comprehensive study about caffeine’s relationship to memory or cognitive function. For starters, participants only drank one cup of coffee with 200 mg of caffeine (about average for a 12-ounce cup). Further research is needed to better understand the mechanisms of how caffeine impacts memory. Sherman says: "If people learn with caffeine and are tested with caffeine, does that make [their memory] is better? What if we tell people they are about to do a memory test and they have caffeine—will that be better? If we have people learn with caffeine and bring them in a week later, will we see an increase in performance? We don’t know the extent."

Similarly, they can’t speak to whether more coffee would increase memory performance rates, since participants only ingested one cup of coffee. "We don’t want anyone to get the idea that we should ingest five cups of coffee to get five times the performance," she cautions.

In short, when it comes to giving your memory a performance jolt, if you’re a college student taking an early exam, you may want to brew up a cup. But if you're over college age, late afternoon may be the best time for that coffee boost.