Caffeine has been shown to enhance several different modes of exercise performance including endurance [8, 16, 25–28], high-intensity team sport activity [29–34], and strength-power performance [30, 35]. Additionally, the use of caffeine has also been studied for its contribution to special force operations, which routinely require military personnel to undergo periods of sustained vigilance and wakefulness. In a series of investigations, McLellan et al. [36–38] examined the effects of caffeine in special force military units who routinely undergo training and real life operations in sleep deprived conditions, where alertness and diligent observation are crucial to assignment.
In the McLellan et al. investigations [36–38], soldiers performed a series of tasks over several days, where opportunities for sleep were exceedingly diminished. Experimental challenges included a 4 or 6.3 km run, as well as tests for marksmanship, observation and reconnaissance, and psychomotor vigilance [36–38]. During periods of sustained wakefulness, subjects were provided caffeine in the range of 600-800 mg, and in the form of chewing gum. The caffeine supplement was consumed in this manner as it has been shown to be more readily absorbed, than if it was provided within a pill based on the proximity to the buccal tissue . In all three studies [36–38], vigilance was either maintained or enhanced for caffeine conditions in comparison to placebo. Additionally, physical performance measures such as run times and completion of an obstacle course were also improved by the effects of caffeine consumption [36, 38].
Lieberman et al.  examined the effects of caffeine on cognitive performance during sleep deprivation in U.S. Navy Seals . However, in this investigation  the participants were randomly assigned varying doses of caffeine in capsule form delivering either 100, 200, or 300 mg. In a manner similar to previous investigations, participants received either the caffeine or placebo treatment and one hour post consumption performed a battery of assessments related to vigilance, reaction time, working memory, and motor learning and memory. In addition, the participants were evaluated at eight hours post consumption to assess duration of treatment effect in parallel to the half-life of caffeine, in a manner similar to a study conducted by Bell et al. .
As to be expected, caffeine had the most significant effect on tasks related to alertness . However, results were also significant for assessments related to vigilance and choice reaction time for those participants who received the caffeine treatment. Of particular importance are the post-hoc results for the 200 and 300 mg doses. Specifically, there was no statistical advantage for consuming 300, as opposed to 200 mg . In other words, those trainees who received the 300 mg (~4 mg/kg) dose did not perform significantly better than those participants who received 200 mg (~2.5 mg/kg). Meanwhile, a 200 mg dose did result in significant improvements in performance, as compared to 100 mg. In fact, it was evident from post-hoc results that 100 mg was at no point statistically different or more advantageous for performance than a placebo. These studies [36–38, 40] demonstrate the effects of caffeine on vigilance and reaction time in a sleep deprived state, in a distinct and highly trained population. These findings suggest that the general population may benefit from similar effects of caffeine, but at moderate dosages in somewhat similar conditions where sleep is limited.
An additional outcome of the Lieberman et al.  study is the fact that caffeine continued to enhance performance in terms of repeated acquisition (assessment of motor learning and short-term memory) and Profile of Mood States fatigue eight hours following consumption. These results are in agreement with Bell et al. , where aerobic capacity was assessed 1, 3, and 6 hours following caffeine consumption (6 mg/kg). Caffeine had a positive effect on performance for participants classified as users(≥ 300 mg/d) and nonusers (≤ 50 mg/d); however, nonusers had a treatment effect at 6 hours post-consumption, which was not the case for users - this group only had a significant increase in performance at 1 and 3 hours post- consumption. Taken together, results of these studies [40, 41] provide some indication, as well as application for the general consumer and athlete. Specifically, while caffeine is said to have a half-life of 2.5-10 hours , it is possible performance-enhancing effects may extend beyond that time point as individual response and habituation among consumers varies greatly.
Finally, it was suggested by Lieberman and colleagues  that the performance-enhancing effects of caffeine supplementation on motor learning and short-term memory may be related to an increased ability to sustain concentration, as opposed to an actual effect on working memory. Lieberman et al.  attributed the effects of caffeine to actions on the central nervous system, specifically the supplement's ability to modulate inhibitory actions, especially those of adenosine. In fact, it was suggested that because caffeine has the ability to act as an antagonist to adenosine, alterations in arousal would explain the compound's discriminatory effect on behaviors relating vigilance, fatigue and alertness .
Recently, it was also suggested that caffeine can positively affect both cognitive and endurance performance . Trained cyclists, who were moderate caffeine consumers (approximated at 170 mg/d) participated in three experimental trials consisting of 150 min of cycling at 60% VO2max followed by five minutes of rest and then a ride to exhaustion at 75% VO2max. On three separate days, subjects consumed a commercially available performance bar that contained either 44.9 g of carbohydrates and 100 mg of caffeine, non-caffeinated-carbohydrate and isocaloric, or flavored water. Results from a repeated series of cognitive function tests favored the caffeine treatment in that subjects performed significantly faster during both the Stroop and Rapid Visual Information Processing Task following 140 min of submaximal cycling as well as after a ride to exhaustion. In addition, participant time increased for the ride to exhaustion on the caffeine treatment, as compared to both the non-caffeinated bar and flavored water .
Overall, the literature examining the effects of caffeine on anaerobic exercise is equivocal, with some studies reporting a benefit [29–32, 43, 44] and others suggesting that caffeine provides no significant advantage [45, 46]. As with all sports nutrition research, results can vary depending on the protocol used, and in particular, the training status of the athlete as well as intensity and duration of exercise. For example, Crowe et al.  examined the effects of caffeine at a dose of 6 mg/kg on cognitive parameters in recreationally active team sport individuals, who performed two maximal 60-second bouts of cycling on an air-braked cycle ergometer. In this investigation , untrained, moderately habituated (80-200 mg/d) participants completed three trials (caffeine, placebo, control) and underwent cognitive assessments prior to consumption of each treatment, post-ingestion at approximately 72-90 min, and immediately following exercise. Cognitive testing consisted of simple visual reaction time and number recall tests. Participants performed two 60-second maximal cycle tests interspersed by three min of passive rest. The results were in contrast to other studies that investigated cognitive parameters and the use of caffeine [25, 36–38, 40] in that caffeine had no significant impact on reaction time or number recall, and there was no additional benefit for measurements of power. In fact, in this study , the caffeine treatment resulted in significantly slower times to reach peak power in the second bout of maximal cycling.
Elsewhere, Foskett and colleagues  investigated the potential benefits of caffeine on cognitive parameters and intermittent sprint activity and determined that a moderate dose (6 mg/kg) of caffeine improved a soccer player's ball passing accuracy and control, thereby attributing the increase in accuracy to an enhancement of fine motor skills.
Based on some of the research cited above, it appears that caffeine is an effective ergogenic aid for individuals either involved in special force military units or who may routinely undergo stress including, but not limited to, extended periods of sleep deprivation. Caffeine in these conditions has been shown to enhance cognitive parameters of concentration and alertness. It has been shown that caffeine may also benefit endurance athletes both physically and cognitively. However, the research is conflicting when extrapolating the benefits of caffeine to cognition and shorter bouts of high-intensity exercise. A discussion will follow examining the effects of caffeine and high-intensity exercise in trained and non-trained individuals, which may partially explain a difference in the literature as it pertains to short-term high-intensity exercise.