Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a psychoactive stimulant drug. It can have both positive and negative effects on different aspects of memory.
Short-term memory
The effects of caffeine on short-term memory (STM) are controversial. Findings are inconsistent, as many effects of caffeine sometimes impair short-term and working memory, whereas the other studies indicate enhancing effects. Increasing our capacities of STM and working memory only seem to have beneficial impacts upon our daily lives. Increasing our memory capacities would result in retaining more information for extended periods of time and encoding information from STM to long-term memory. However, the research consensus indicates an inhibitory effect, reducing the capacity of our short-term memory and working memory.
Auditory effects within short-term memory
Caffeine’s effects in memory were also investigated in the auditory system. The Auditory-Verbal Learning Test is a memory test that assesses recall of lists of words on single and multiple trials given through the subjects' auditory system. Caffeine subjects recalled fewer words than did control subjects, and caffeine subjects showed a greater deficit in recalling the middle- to end-portions of the lists.
Working memory effects
Caffeine has been thought to have some benefits when testing working memory by investigating the tip of the tongue effect, the idea being that, if caffeine were present in one's system, then one would be less likely to experience tip of the tongue effect, or the feeling of knowing a familiar word but not being able to immediately recall it. Previous research suggested that the tip of the tongue phenomena can be corrected for with the use of caffeine, and that caffeine could help one to more quickly retrieve the word they are looking for. Current research refutes previous research accrediting evidence to priming a phonological loop within the working memory opposed to caffeine enhancing STM capacity. A study has found that there are more correct tip of the tongue answers with a caffeine group than those of the control group. The finding is not that caffeine improves performance; it was finding the priming of the phonological-system-effecting responses. When attempting to comprise tip of the tongue effects, subjects were primed with similar-sounding words to the target word; as a result, priming the target word was reached faster regardless of caffeine intake.
A test also studied the effects of caffeine on working memory by administering word lists to subjects. Ultimately, the study found that there was a difference in the results for male and female subjects. Male subjects saw no change in their ability to recall the word lists, thus caffeine showed no effect on their working memory, while females under the effects of the caffeine supplement saw their ability to recall the word lists negatively effected and thus their working memories slightly impaired by the drug
Time-of-day effects
Short-term memory has been thought to be influenced differently throughout the day when caffeine has been ingested; in the morning, STM performance will be different from at the end of the day. As the effects of caffeine wear off, there would be some effect on STM. Three groups of caffeine intake (low, medium, and high) were compared during four daytime hours (01:00, 07:00, 13:00, 19:00). People with low caffeine intake have a decreased performance later in the day, compared to moderate and a high-level caffeine intake. Thus the effect of caffeine on short term memory can differ due to many other factors and thus cannot be instantly generalized.
State Dependent Memory
Caffeine users are subject to state dependent memory effects when under the effects of caffeine. For example, a study tasked two groups of subjects to remember word lists; half of them had caffeine while the other half were given placebos. When brought back the next day, each original group was now split in half, half of them given the same treatment they were encoded under, the other half given the opposite treatment. Ultimately the study showed that subjects that were under the same treatment in both encoding and retrieval out performed the other groups. This study does not support a decline or enhancement in working memory due to caffeine but rather a state dependent effect instead.
Long-term memory
Caffeine has been shown to have positive, negative, and no effects on long-term memory. When studying the effects of this and any drug, potential ethical restraints on human study procedures may lead researchers to conduct studies involving animal subjects in addition to human subjects.
Positive effects of caffeine on long-term memory
Positive effects of caffeine on long-term memory have been shown in a study analyzing habitual caffeine intake of coffee or tea in addition to consuming other substances. Their effect on cognitive processes was observed by performing numerous cognitive tasks. Words were presented and delayed recall was measured. Increased delayed recall was demonstrated by individuals with moderate to high habitual caffeine intake (mean 710 mg/week) as more words were successfully recalled compared to those with low habitual caffeine intake (mean 178 mg/week). Therefore, improved performance in long-term memory was shown with increased habitual caffeine intake due to better storage or retrieval. A similar study assessing effects of caffeine on cognition and mood resulted in improved delayed recall with caffeine intake. A dose-response relationship was seen as individuals were able to recall more words after a period of time with increased caffeine. Improvement of long-term memory with caffeine intake was also seen in a study using rats and a water maze. In this study, completion of training sessions prior to performing numerous trials in finding a platform in the water maze was observed. Caffeine was consumed by the rats before and after the training sessions. There was no effect of caffeine consumption before the training sessions; however, a greater effect was seen at a low dosage immediately afterward. In other words, the rats were able to find the platform faster when caffeine was consumed after the training sessions rather than before. This implies that memory acquisition was not affected, while increases in memory retention were.
Negative effects of caffeine on long-term memory
Researchers have found that long-term consumption of low dose caffeine slowed hippocampus-dependent learning and impaired long-term memory in mice. Caffeine consumption for 4 weeks also significantly reduced hippocampal neurogenesis, a process by which the brain creates new neurons to assist in memory retention, compared to controls during the experiment. The conclusion was that long-term consumption of caffeine could inhibit hippocampus-dependent learning and memory partially through inhibition of hippocampal neurogenesis.
Caffeine has been shown to have negative effects on long-term memory. In a study with mice, a step-through passive-avoidance task was used, and the ability to remember the task was assessed. Caffeine was given before the task in varying doses, with low doses to start (11.55 mg/kg) and higher doses in the end (92.4 mg/kg). (To put that in perspective, one 8 oz cup of coffee contains 95–200 mg of caffeine.) An apparatus including a box with a light was connected to a dark box with an electric floor. When the mice entered the dark box, a shock was released from the floor. The next day, the mice entered the apparatus again and completed the same task. Subjects that did not enter the dark box for 180 seconds were thought to have put the task in long-term memory, remembering the assignment from the previous day. However, caffeine administered at higher doses resulted in decreased retention time of the task from 180 seconds to 105 seconds. Lower doses of caffeine had little to no effect on retention time. Therefore, in this study, linear regression analysis showed that higher doses of caffeine impaired long-term memory, suggesting a dose-response relationship between caffeine intake and retention time. Ultimately, long-term memory and caffeine intake display varying results, in human as well as animal subjects.
No effect of caffeine on long-term memory
Alternatively, other studies have shown that caffeine intake has no effect on long-term memory. This was expressed in a study whereby either caffeine or a placebo was assigned to several subjects at two different times. Some subjects received caffeine first, while others received a placebo. All participants were shown a word list which would eventually be tested. Two days later, the same process was repeated, with random distribution of the two substances. This was also observed in a study involving the assessment of delayed recall using a verbal memory test. Two studies were completed using different control drinks containing caffeine.
Age differences
Effects on young adults
The effects for this age group (15-25) were the most variable and conflicting. On the one hand, caffeine effects appear to be detrimental to short-term memory, working memory included, whereas the effects are somewhat positive for memory over the long term (for example, you remember something better many days later if you drank caffeine during encoding as well as retrieval, as opposed to no caffeine). Many of the effects reported were for subjects who were not regular caffeine consumers. Regular consumers of caffeine, on the other hand, showed only positive effects when it came to memory tasks. An important factor to consider is that there was fairly wide-range daily caffeine consumption previous to the study, and this could have had a significant effect on performance of the task because not everyone is at the same baseline. Another study used a much larger subject pool and found that age-related differences were quite minimal for attentional memory, but that over the long term, regular caffeine consumption was fairly beneficial to younger subjects.
Effects on the middle aged
As previously stated, the most pronounced effect of caffeine on memory appears to be on middle-aged subjects (26-64). None of the studies provide reasoning for why this group would be most affected, but one could hypothesize that because of cognitive decline due to age, caffeine has a powerful effect on brain chemistry (although this would suggest the older the person, the stronger the effect of caffeine). Furthermore, this age group is most likely to be the largest consumer of caffeine. The main studies reporting this finding show that at low, acute doses of caffeine consumption, working memory only slightly affects those in this age group, while no effect is observed for younger or older subjects. The authors conclude that larger doses may be needed to produce results that are supported by previous literature, and this is an avenue for further research. Furthermore, it is argued that consumption of caffeine generally aids cognitive performance for this age group, as long one does not exceed the recommended dose of 300 mg per day.
Effects on the elderly
In older adults, memory is typically best in the morning and gradually declines over the day. Those who consumed caffeine in the morning showed much better memory, both short-term and long-term than those who consumed a placebo, especially in late afternoon, where memory and attention may be most crucial to daily functioning for the elderly. This is further supported by a study which showed that adults over the age of 65 who regularly consume caffeine in the morning are much more alert and function at a higher cognitive level throughout the day. The authors conclude that it is beneficial for older adults to regularly consume average doses of caffeine in the morning to boost cognitive performance and alertness in the afternoon. Again, one should not exceed the recommended dose of about 300 mg per day, otherwise memory performance declines due to over-consumption.
Conclusion
The literature shows mixed results. Overall regular caffeine consumption has a positive effect on one's short- and long-term memory only when consumed on a regular basis. Consumption should be daily, in moderate doses, and at around the same time, regardless of age.
Sex differences
Many studies provide support for the idea that caffeine has different effect on males versus females when related to memory. These differences can be seen through a number of memory types (short-term, long-term, etc.), with various theories accounting for these differing effects.
Short-term memory
Caffeine has been shown to have an impairing effect on females (but not males) in a word-list test of short-term memory. One prevailing theory which aims to explain this sex difference identifies estrogen levels in the body as an important factor relating to caffeine’s effect on memory performance As a result, the female menstrual cycle (which influences overall estrogen levels in the body) may play a role in modifying the effect of caffeine on memory. Following this theory, researchers tested females within the first 5 days of their menstrual cycle and found that caffeine had a facilitative effect on female performance on a short-term memory test. A particular finding in this study relating to male memory performance revealed that at a lower dose, caffeine had an impairing effect; but at higher doses, no impairment was shown. Differing speeds of testing (words delivered slowly or quickly) in males served as a modifying factor on the effect of caffeine: higher doses aided in recall with faster presentation of words, and lower doses aided in recall with slower presentation of words. These findings are only based on a small set of data collected from selective studies on this topic, so further research in this area would be needed to gain a more clear understanding of caffeine's differing effects on male and female short-term memory.
Long-term memory
Limited research on long-term memory and sex differences indicates no notable difference in the effect of caffeine between males and females. Sex differences have not been thoroughly covered in the literature concerning caffeine’s effect on memory. Since most studies do not report significant sex differences in this area of memory study, it is reasonable to assume that there is not strong evidence to support sex differences in caffeine’s effect on memory. Further specific research into sex differences would be needed to fully understand the impacts of this popular drug on its users.
Withdrawal
Caffeine withdrawal has been known about for over a hundred years. However, there are still many unknowns that exist because only within the last decade has it been researched scientifically. Currently, there is no known correlation between caffeine withdrawal and an effect on memory. There are many potential reasons for the lack of conclusions made about this issue. The main speculation is that since caffeine affects many parts of the central nervous system, this would imply that there is more than one mechanism that is activated by caffeine. It would thus require the examination of multiple activation pathways in order to determine caffeine’s specific effect on the nervous system and consequently memory.
Caffeine withdrawal's physiological effects
Even though there is no direct evidence that caffeine withdrawal impacts memory, there are many other connections made that provide some insight into what memory effects are possible. For example, there is evidence to show that attention decreases when experiencing caffeine withdrawal. A study had school-age children, who were regular caffeine users, go 24 hours without caffeine consumption, and the results showed a decrease in performance on reaction time of a task that required attention. Studies have also shown that regular caffeine users experience headaches and fatigue during withdrawal. One study had a group of regular caffeine users divided into three groups. Each group was designated an amount of time to avoid caffeinated products, for either 1.5 hours, 13 hours, or 7 days. The study found that, to varying degrees, all participants experienced an increase in fatigue, drowsiness, and headaches. A third study also found that among a group of participants receiving a placebo, anxiety was experienced by participants who had prior caffeine use. This would imply that participants would also experience a deficit in memory capabilities because attention and alertness positively impact the amount of information that can be stored in both short- and long-term memory, and anxiety would be a detriment to memory retention.
Duration of caffeine avoidance
There is also existing evidence that reflects on the duration of the caffeine avoidance period in relation to the significance of the withdrawal symptoms. In the study previously mentioned, the strongest withdrawal effects were seen among participants who underwent a 13-hour avoidance period, followed by the 7-day avoidance group. This would imply that memory effects would be at their strongest around the 13-hour mark and would continue to be affected for the following days. Memory would not be affected, however, within the first few hours. This appears valid considering most daily caffeine users need to consume caffeine shortly after awaking from sleep. For example, coffee drinkers were given either caffeine or a placebo after overnight caffeine abstinence. The study showed that regular coffee drinkers became less alert and more anxious than non-coffee drinkers when receiving the placebo. To coincide with this finding, another study in 2001 found a dose-related improvement in cognitive performance for daily caffeine users. This means that coffee drinkers experience the same positive effects every day they consume coffee.
