The effects of caffeine, impulsivity and time of day on performance, mood and cardiovascular function

Prospective Memory, Sustained Attention and Response Inhibition in Poly-Substance Users Stable on Methadone Maintenance Treatment

Background: Prospective memory and response inhibition are interrelated constructs, though studied separately in the drug addiction literature. Also, although sustained attention underlies response inhibition, its role in the relation between these functions has been largely neglected. The limited research on the cognitive effects of methadone-maintenance treatment (MMT) further stresses the importance of investigating these effects. Objective: Therefore, the current study focused on possible effects of MMT combined with long-term drug abuse on all these functions. Thirty five long term opiate/poly-substance users in MMT and thirty four drug-free controls were screened for socioeconomic status, anxiety, depression and general, non-verbal intelligence and then tested on a self-report measure of prospective memory and on the Go/No-Go task. Results: Compared to controls, the MMT group scored worse in all functions assessed. Prospective memory scores were also negatively related to Go/NoGo accuracy scores. Conclusion: As predicted, (a) the MMT participants show impairments in prospective memory, sustained attention and response inhibition and (b) prospective memory, response inhibition and sustained attention are related constructs. The results of this study could inform current rehabilitation and relapse prevention cognitive training practices.

The impact of sleep deprivation and alcohol on driving: a comparative study

Background

There is concern about the detrimental effects of shift-workers’ increasing working hours particularly when driving sleep deprived. The approach to measuring the magnitude of driving impairment caused by sleep deprivation was by comparing it to alcohol. The study compared driving performance after 24-h of wakefulness to performance with a BrAC of just over 22 μg/100mls of breath which is equal to 50 mg of alcohol per 100mls of blood (Scottish drink-drive limit). The effectiveness of coffee as a countermeasure for driver fatigue and the association between subjective impairment and actual performance was also investigated.

Methods

A study of 30 participants (11 male and 19 female; mean age 21) was conducted. Subjects were tested under three conditions: fully rested, sleep deprived, and alcohol intoxicated – BrAC mean [SD] 25.95 μg [2.78]. Under each condition, subjects were tested before and after coffee ingestion. This involved driving simulation (Lane Change Task and Reaction Test) and subjective Likert scales (Karolinska Sleepiness Scale and driver impairment scale). Outcome measures included lane tracking adaptive mean deviation, reaction time, and subjective sleepiness and impairment ratings.

Results

Compared to alcohol, sleep deprived mean reaction times were slower (2.86 s vs. 2.34 s) and lateral control of the vehicle was reduced (lane tracking adaptive mean deviation: 0.5 vs. 0.3). Coffee did not produce an improvement when sleep deprived, and instead, performance deteriorated. Females were less impaired following sleep deprivation than males. Following prolonged wakefulness, the correlation between subjective impairment and actual performance was significant.

Conclusions

It was concluded that sleep deprivation has a greater impact on driving performance than a BrAC of 22 μg/100mls of breath, as measured by driving simulation. Coffee is not an effective countermeasure for sleep deprived driving and drivers’ ability to judge this impairment is suggested to be limited.

The impact of Yohimbine-induced arousal on facets of behavioural impulsivity

RATIONALE

State-dependent changes in physiological arousal may influence impulsive behaviours.

OBJECTIVES

To examine the relationship between arousal and impulsivity, we assessed the effects of yohimbine (an α₂-adrenergic receptor antagonist, which increases physiological arousal via noradrenaline release) on performance on established laboratory-based impulsivity measures in healthy volunteers.

METHODS

Forty-three participants received a single dose of either yohimbine hydrochloride or placebo before completing a battery of impulsivity measures. Blood pressure and heart rate were monitored throughout the study.

RESULTS

Participants in the yohimbine group showed higher blood pressure and better response inhibition in the Stop Signal Task, relative to the placebo group. Additionally, individual changes in blood pressure were associated with performance on Delay Discounting and Information Sampling tasks: raised blood pressure following drug ingestion was associated with more far-sighted decisions in the Delay Discounting Task (lower temporal impulsivity) yet reduced information gathering in the Information Sampling Task (increased reflection impulsivity).

CONCLUSIONS

These results support the notion that impulsive behaviour is dependent upon state physiological arousal; however, distinct facets of impulsivity are differentially affected by physiological changes.

Pramipexole-induced impulsivity in mildparkinsonian rats: a model of impulse control disorders in Parkinson's disease

Treatment with dopaminergic agonists such as pramipexole (PPX) contributes to the development of impulse control disorders (ICDs) in patients with Parkinson's disease (PD). As such, animal models of abnormal impulse control in PD are needed to better study the pathophysiology of these behaviors. Thus, we investigated impulsivity and related behaviors using the 5-choice serial reaction time task, as well as FosB/ΔFosB expression, in rats with mild parkinsonism induced by viral-mediated substantia nigra overexpression of human A53T mutated α-synuclein, and following chronic PPX treatment (0.25 mg/kg/d) for 4 weeks. The bilateral loss of striatal dopamine transporters (64%) increased the premature response rate of these rats, indicating enhanced waiting impulsivity. This behavior persisted in the OFF state after the second week of PPX treatment and it was further exacerbated in the ON state throughout the treatment period. The enhanced rate of premature responses following dopaminergic denervation was positively correlated with the premature response rate following PPX treatment (both in the ON and OFF states). Moreover, the striatal dopaminergic deficit was negatively correlated with the premature response rate at all times (pretreatment, ON and OFF states) and it was positively correlated with the striatal FosB/ΔFosB expression. By contrast, PPX treatment was not associated with changes in compulsivity (perseverative responses rate). This model recapitulates some features of PD with ICD, namely the dopaminergic deficit of early PD and the impulsivity traits provoked by dopaminergic loss in association with PPX treatment, making this model a useful tool to study the pathophysiology of ICDs.

The role of emotions and physiological arousal in modulating impulsive behaviour

Impulsivity received considerable attention in the context of drug misuse and certain neuropsychiatric conditions. Because of its great health and well-being importance, it is crucial to understand factors which modulate impulsive behaviour. As a growing body of literature indicates the role of emotional and physiological states in guiding our actions and decisions, we argue that current affective state and physiological arousal exert a significant influence on behavioural impulsivity. As 'impulsivity' is a heterogeneous concept, in this paper, we review key theories of the topic and summarise information about distinct impulsivity subtypes and their methods of assessment, pointing out to the differences between the various components of the construct. Moreover, we review existing literature on the relationship between emotional states, arousal and impulsive behaviour and suggest directions for future research.

The Influence of Caffeine Arousal on the Frequency of Task-Unrelated Image and Thought Intrusions

Based upon earlier studies relating task-unrelated image and thought (TUIT) frequency to circadian activity levels and to decreases with increased age in adulthood it was hypothesized that increased cortical arousal would result in greater TUIT frequency. In a double-blind study, six young men had arousal induced at two dosages of caffeine, 100 mg and 300 mg. When unintentional and deliberate TUIT frequency at the 100 mg and 300 mg dosages was contrasted with the placebo no significant effects were noted. Change in TUIT frequency between placebo and the 300 mg dosage was significantly correlated with the personality trait of Agreeableness. Lack of compliance, caffeine withdrawal influences, and insufficiently high arousal are discussed as possible sources of the lack of relationship obtained between caffeine dosage and TUIT frequency.

Relationships between Dimensions of Impulsivity and Prospective Memory

Prospective memory refers to the ability to plan and execute future intentions. A burgeoning body of research indicates that a significant proportion of the variability in prospective memory performance can be accounted for by personality traits, with two recent studies revealing that the various dimensions of impulsivity measured by the Barratt Impulsivity Scale–11 (BIS–11) are related to prospective memory failures. The present study was conducted to examine which dimensions of impulsivity indexed by the BIS–11 and UPPS–P relate to prospective memory failures and which dimensions of impulsivity contribute unique variance in the prediction of prospective memory failures. A large sample ( N = 662) of students completed the BIS–11, UPPS–P, Prospective Memory Questionnaire and a habitual prospective memory test. Consistent correlations between six of the seven measures of impulsivity and prospective memory failures were found. Regression analyses further revealed that lack of perseverance, emotional urgency and attentional impulsivity were reliable and unique predictors of prospective memory failures. Thus, difficulty remaining focused on tasks (especially long, boring or difficult tasks); lack of forethought, planning and distaste for challenging tasks; and tendencies to behave impulsively under conditions of strong affect relate most strongly to prospective memory failures. Copyright © 2015 European Association of Personality Psychology

Quick to Act, Quick to Forget: The Link between Impulsiveness and Prospective Memory

Several traits of impulsiveness (e.g. lack of planning and perseverance, difficulty focusing attention) seem intimately connected to the skills required for successful prospective memory performance. This is the first study to examine whether the various inter–correlated dimensions of impulsiveness are related to problems with prospective memory. Undergraduate students (N = 184) completed the Barratt Impulsiveness Scale 11, the Prospective Memory Questionnaire, the Prospective and Retrospective Memory Questionnaire, and two objective prospective memory tests. Results revealed consistent correlations between the various dimensions of impulsiveness (attentional, motor, non–planning) and self–reported problems with prospective memory. Subsequent regression analyses indicated that attentional impulsiveness is a unique predictor of self–reported problems with internally cued prospective memory, and non–planning impulsiveness is a unique predictor of self–reported problems with episodic and overall prospective memory. Similarly, findings from the objective prospective tests showed that non–planning impulsiveness was related to worse performance on the two prospective memory tests. Whereas non–planning impulsiveness was also related to using fewer prospective memory–aiding strategies, mediation analyses showed that use of these strategies does not account for any of the detected relationships. Because the findings suggest that a failure to plan does not underlie the detected effects, other potential explanations for the relationships are discussed. Copyright © 2013 European Association of Personality Psychology

Cumulative neurobehavioral and physiological effects of chronic caffeine intake: individual differences and implications for the use of caffeinated energy products

The use of caffeine-containing energy products has increased worldwide in recent years. All of the top-selling energy drinks contain caffeine, which is likely to be the primary psychoactive ingredient in these products. Research shows that caffeine-containing energy products can improve cognitive and physical performance. Presumably, individuals consume caffeine-containing energy products to counteract feelings of low energy in situations causing tiredness, fatigue, and/or reduced alertness. This review discusses the scientific evidence for sleep loss, circadian phase, sleep inertia, and the time-on-task effect as causes of low energy and summarizes research assessing the efficacy of caffeine to counteract decreased alertness and increased fatigue in such situations. The results of a placebo-controlled experiment in healthy adults who had 3 nights of total sleep deprivation (with or without 2-hour naps every 12 hours) are presented to illustrate the physiological and neurobehavioral effects of sustained low-dose caffeine. Individual differences, including genetic factors, in the response to caffeine and to sleep loss are discussed. The review concludes with future directions for research on this important and evolving topic.

Caffeine, extraversion and working memory

Research has shown that extraverts performing a working memory task benefit more from caffeine than do introverts. The present study aimed to replicate this and extend our knowledge by using a lower dose of caffeine (65 mg) and a range of tasks related to different components of working memory. In addition, tasks assessing psychomotor speed and the encoding of new information were included to determine whether caffeine-extraversion interactions were restricted to working memory tasks. A double-blind design was used, with 128 participants being randomly assigned to caffeinated or de-caffeinated coffee conditions. The results showed that caffeine interacted with extraversion in the predicted direction for serial recall and running memory tasks. Caffeine improved simple reaction time and the speed of encoding of new information, effects which were not modified by extraversion. These results suggest possible biological mechanisms underlying effects of caffeine on cognitive performance.

Effects of slow-release caffeine and a nap on driving simulator performance after partial sleep deprivation

A driving simulator was used to evaluate the effectiveness of a 30-min. nap and 300-mg slow-release caffeine as countermeasures to drivers' sleepiness induced by partial sleep deprivation. 12 participants were allowed 45 hr. time in bed at the laboratory. Driving performance then was measured twice--at 9 a.m. and at 1 p.m.--by a 45-min. driving task on a simulator. Subjective sleepiness/alertness and mood were assessed four times on the Stanford Sleepiness Scale and the Profile of Mood States. Driving performance was assessed as Lane Drifting, Speed Deviation, and Accident Liability. A 30-min. nap opportunity and 300 mg of slow-release caffeine both were successful in counteracting drivers' sleepiness. The remedial effect of slow-release caffeine lasted longer than that of the nap, that is, it was also effective in the afternoon session. This suggests that slow-release caffeine represents a valuable countermeasure that, in the case of partial sleep deprivation, is preferred to a nap when sleepiness has to be counteracted for a longer time.

Effects of caffeine on human behavior

The literature suggests that the following effects on behavior of adult humans may occur when individuals consume moderate amounts of caffeine. (1) Caffeine increases alertness and reduces fatigue. This may be especially important in low arousal situations (e.g. working at night). (2) Caffeine improves performance on vigilance tasks and simple tasks that require sustained response. Again, these effects are often clearest when alertness is reduced, although there is evidence that benefits may still occur when the person is unimpaired. (3) Effects on more complex tasks are difficult to assess and probably involve interactions between the caffeine and other variables which increase alertness (e.g. personality and time of day). (4) In contrast to the effects of caffeine consumption, withdrawal of caffeine has few effects on performance. There is often an increase in negative mood following withdrawal of caffeine, but such effects may largely reflect the expectancies of the volunteers and the failure to conduct "blind" studies. (5) Regular caffeine usage appears to be beneficial, with higher users having better mental functioning. (6) Most people are very good at controlling their caffeine consumption to maximise the above positive effects. For example, the pattern of consumption over the day shows that caffeine is often consumed to increase alertness. Indeed, many people do not consume much caffeine later in the day since it is important not to be alert when one goes to sleep. In contrast to effects found from normal caffeine intake, there are reports that have demonstrated negative effects when very large amounts are given or sensitive groups (e.g. patients with anxiety disorders) were studied. In this context caffeine has been shown to increase anxiety and impair sleep. There is also some evidence that fine motor control may be impaired as a function of the increase in anxiety. Overall, the global picture that emerges depends on whether one focuses on effects that are likely to be present when caffeine is consumed in moderation by the majority of the population or on the effects found in extreme conditions. The evidence clearly shows that levels of caffeine consumed by most people have largely positive effects on behavior. Excessive consumption can lead to problems, especially in sensitive individuals.

The effects of caffeine on simulated driving, subjective alertness and sustained attention

There is evidence that caffeine increases alertness and reduces fatigue. This may be especially so in low arousal situations (e.g. working at night or for prolonged hours). Caffeine has also been found to improve performance on vigilance tasks and simple tasks requiring sustained response. Again, these effects are often clearest when alertness is reduced, although there is evidence that benefits may still occur when the individual is unimpaired. Most studies to date have investigated the behavioural effects of caffeine in laboratory experiments using artificial tasks. In the current study 3 mg/kg caffeine was found to improve steering accuracy in a 1 h simulated drive. Measures of mood and performance on a sustained attention task also showed the benefits of caffeine. These findings suggest that laboratory results reflect a general benefit of caffeine that may also be observed in real-life situations. Other evidence examining the effects of caffeine on performance efficiency over the working day has shown the benefits of caffeine consumption on measures of sustained attention and alertness. This study also provided evidence suggesting that caffeine is often consumed when alertness is low to maximise alertness and performance efficiency. The implications of these findings for road safety are also considered. Copyright 2001 John Wiley & Sons, Ltd.

Slow-release caffeine as a countermeasure to driver sleepiness induced by partial sleep deprivation

The effect of partial sleep deprivation (PSD) on driving abilities, as measured with a driving simulator, and the value of slow-release caffeine as a countermeasure to the expected performance decrements, were studied. Twelve subjects, between 20 and 25 years of age, underwent four experimental conditions, 4.5 or 7.5 h time in bed (TIB) with 300 mg slow-release caffeine or placebo, according to a Latin square design. Driving performance was measured twice by a 45-min driving task on a simulator. Subjective sleepiness/alertness and mood were assessed four times, by means of the Stanford Sleepiness Scale (SSS) and Profile of Mood States (POMS). After 4.5 h as compared with 7.5 h TIB lane drifting and speed deviation were higher, but only the effect on the first variable reached significance. In the placebo condition at 13.00 h, accident liability increased after PSD. Subjective sleepiness was higher in the 4.5 h TIB group. Caffeine intake gave rise to a decrease in lane drifting and after PSD it led to a smaller speed deviation and accident liability. The findings suggest that a lack of sleep can lead to a significant driving performance impairment, with drivers having problems to maintain an appropriate road position and a posted speed and more drivers getting involved in an accident. Secondly, the results indicate that caffeine - more specifically slow-release caffeine - can serve as a valuable countermeasure to these performance decrements, in the absence of any important side-effects, especially when its application is of an acute nature and when there is no opportunity to take a nap.

Functional food science and behaviour and psychological functions

The impact of ingesting various foods on psychological and behavioural functions is a topic of both interest and concern to the general public. In this article, the scientific literature concerning demonstrated cause-and-effect relationships is reviewed, beginning with methodological considerations specific to the quantification of particular behaviours and psychological events. The essential function of food is to satisfy hunger and the need for essential nutrients. The contributions of macronutrients to appetite and satiety are described, as well as their impact on metabolism and energy balance. Functional properties of macronutrient substitutes (high-intensity sweeteners, fat replacers) and flavour enhancers are examined in relation to their contribution to hunger, satiety, and energy balance. The effects of foods and individual nutrients on the performance of diverse psychomotor tasks are studied with consideration given to the various validated quantitative tools used to assess behaviour. The effects of food components on activation, sedation, and affective states such as dysphoria are also reviewed, with special attention given to brain function and neuroactive substances such as serotonin and the endorphins. The case of hyperactivity in children is given special emphasis with reference to the potential influence of sugar and food additives. Safety issues related to food constituents and additives are discussed. Finally, a set of criteria is proposed for the evaluation and elaboration of studies in the behavioural and psychological fields, along with suggestions for future research.

Regular caffeine consumption: a balance of adverse and beneficial effects for mood and psychomotor performance

It has often been pointed out that caffeine is the most widely "used" psychoactive substance in the world, and accordingly, there is a very large amount of research available on the effects of caffeine on body and mind. In particular, a psychostimulant action of caffeine is generally accepted as well established; for example, caffeine has been found to quicken reaction time and enhance vigilance performance, and to increase self-rated alertness and improve mood. There is, however, a real difficulty in determining the net effects of caffeine. In a typical experiment the subjects have a history of regular caffeine consumption, and they are tested on caffeine and a placebo after a period of caffeine deprivation (often overnight). The problem with relying solely on this approach is that it leaves open the question as to whether the results obtained are due to beneficial effects of caffeine or to deleterious effects of caffeine deprivation. The present article briefly reviews this evidence on the psychostimulant effects of caffeine, and presents some new data testing the hypothesis that caffeine may enhance cognitive performance to a greater extent in older adults than in young adults. No age-related differences in the effects of caffeine on psychomotor performance were found. We conclude that overall there is little unequivocal evidence to show that regular caffeine use is likely to substantially benefit mood or performance. Indeed, one of the significant factors motivating caffeine consumption appears to be "withdrawal relief."

Gender and impulsivity differences in licit substance use

Gender differences in ratings of impulsivity have shown increased impulsivity in men. Research also suggests higher impulsivity is correlated with increased use of alcohol, nicotine, and caffeine. In a large sample of young adults (N = 332), self-report data were collected on licit substance use and impulsivity/nonconformity. Among men, significant differences were found between higher and lower impulsivity groups for alcohol and caffeine use but not for nicotine use. Among women, significant differences were found between higher and lower impulsivity groups for alcohol and nicotine use but not for caffeine use. The data suggest the importance of recognizing gender differences in the relationship between personality factors and licit substance use.

Overnight caffeine abstinence and negative reinforcement of preference for caffeine-containing drinks

It has been suggested that liking for the taste, flavour and aroma of, for example, coffee and tea is acquired through the process of classical conditioning, involving association of these orosensory cues with the psychopharmacological consequences of caffeine ingestion. Accordingly, this study investigated caffeine reinforcement by assessing changes in preference for a novel drink consumed with or without caffeine. Particular care was taken to use "ecologically valid" procedures; that is, overnight caffeine abstinence followed by a cup-of-coffee equivalent dose of caffeine (70 mg) at breakfast. Caffeine had no significant effects on drink preference or mood in subjects with habitually low intakes of caffeine. In contrast, moderate users of caffeine developed a relative dislike for the drink lacking caffeine and showed somewhat lowered mood following overnight caffeine abstinence (e.g., less lively, clearheaded and cheerful), which was significantly improved by caffeine. These together with other recent results strongly suggest that, in everyday life, caffeine reinforcement can occur as the result of the alleviation by caffeine of the adverse effects of overnight caffeine abstinence (negative reinforcement). They also demonstrate the utility of this flavour-conditioning procedure, which could be applied in the wider investigation of the reinforcing properties of drugs.

Caffeine and time of day effects on a force discrimination task in humans

The effects of caffeine (0 mg/kg, 1 mg/kg, 3 mg/kg) and time of day (TOD) on human performance were studied using a multiple forceband discrimination task (MFDT) and subjective ratings. Self-rated measures of energy level were affected by TOD and caffeine, while mood was affected by TOD. Energy level decreased throughout the day and was offset by caffeine which increased energy level independent of TOD. Self-reported anxiety was not affected by TOD or caffeine. Mood was affected by TOD in a complex cubic trend with late morning and late evening peaks 12 h apart. MFDT performance was affected by TOD, caffeine dosage, and their interaction. Trend analyses showed varying patterns of TOD effects across peak force variability, response latency, response duration, and correct responding. Results support and extend previous findings of TOD influences on the MFDT and support the utility of multicomponent proprioceptive tasks for examining drug effects on performance.

A comparison of the acute effects of a low dose of alcohol on mood and performance of healthy volunteers and subjects with upper respiratory tract illnesses

An experiment was carried out to determine whether a low dose of alcohol produced different behavioural effects in healthy volunteers to those suffering from an upper respiratory tract illness. Ninety-nine subjects were tested, with 48 subjects being assigned to the alcohol condition and 51 to the juice only condition. A dose of 1.5 ml of vodka per kg body weight was used and the alcohol manipulation was double-blind. Approximately half of the subjects in each condition were healthy and the others had upper respiratory tract illnesses, probably colds. Subjects with colds reported an increase in negative affect and were slower at performing psychomotor tasks. Few main effects of alcohol were obtained. Of major interest were the interactions between health status and alcohol conditions. The alcohol improved the mood of healthy subjects but produced greater negative moods in subjects with colds. Similarly, performance of selective and sustained attention tasks showed different effects of alcohol in healthy and ill subjects.

Impairment of cognitive performance associated with dieting and high levels of dietary restraint

Seventy women students were tested on a short battery of tasks assessing cognitive performance. They also completed self-report ratings of mood, the Dutch Eating Behaviour Questionnaire (DEBQ) to which was appended several additional items concerning their recent dieting behaviour, and a 24-h dietary recall. Heart rate was measured before and after testing. Compared with nondieting subjects with low to moderate scores on the restraint factor of the DEBQ, subjects (n = 15) who reported that they were currently dieting to lose weight displayed impaired performance on a vigilance task and also tended to show poorer immediate memory and longer reaction times. Highly restrained eaters who were not dieting at the time of testing, on the whole, performed at an intermediate level on these tests. In contrast, the dieters tended to show the best performance on an undemanding finger tapping task, indicating that they were not slowed in their fine motor responses or lacking in motivation to carry out the tasks. Poorer cognitive functioning during dieting could arise as a direct consequence of the effects of food restriction on energy metabolism or other physiological mechanisms--the dietary records indicated that the current dieters were eating at about 70% of maintenance energy requirement. However, it is also possible that cognitive performance is impaired during dieting due to anxiety resulting from stressful effects of imposing and maintaining dietary restraint.

The effects of caffeine and evening meals on sleep and performance, mood and cardiovascular functioning the following day

An experiment was carried out to examine the effects of caffeine and evening meals on sleep and performance, mood and cardiovascular functioning in the early morning of the next day. Forty-eight subjects were assigned to one of the four conditions formed by combining caffeine and meal conditions. Subjects in the caffeine condition were given 3 mg/kg caffeine in de-caffeinated coffee. The caffeine manipulation was double blind. Subjects in the meal condition were given a 3-course meal (∼ 1300 calories). Sleep was assessed by subjective ratings and these showed that both caffeine and consumption of the meal influenced sleep, but that there were no interactions between caffeine and meal conditions. Although caffeine disrupted sleep there was no evidence of performance or mood being impaired the next day. However, blood pressure was still higher the next day in subjects given caffeine the previous evening.

Effects of caffeine, time of day and user history on study-related performance

The individual and interactive effects of caffeine, time of day and history of caffeine consumption on several study-related tasks were investigated in 25 subjects (6 males, 19 females). Performance was measured on short term memory (STM), mental arithmetic (MA), reading comprehension, serial search (SS) and verbal reasoning (VR). Subjects attended eight experimental sessions, at four times of day (0100, 0700, 1300 and 1900 hours), after ingesting caffeine (4 mg/kg) or placebo. Subjects were assigned to a low, moderate or high user group on the basis of a caffeine consumption questionnaire. Reading comprehension was affected by time of day, while caffeine improved performance on all mental speed-related tasks. High caffeine users performed more poorly than other groups on the verbal reasoning task. Several interactions between the three independent variables were observed on a number of tasks, supporting the contention that different processes underlying various types of cognitive performance are differentially, and often jointly, affected by caffeine, time of day and user history. Implications of caffeine usage on academic performance were discussed.