Caffeine has a number of different pathways for action, but the main way in which it can wake you up lies in the fact that it blocks adenosine receptors in the brain, effectively disrupting the homeostatic regulation of sleep pressure (check back to the sleep module for more details on this).
The binding of caffeine to adenosine receptors also causes the secretion of the neurotransmitter acetylcholine, responsible for stimulating areas of the autonomic nervous system to increase heart rate and blood pressure, and other neurotransmitters involved with increasing arousal – this all makes you more able to remain alert physically as well as mentally. In addition, it disrupts the secretion of the ‘relaxing’ neurotransmitter known as Gamma-Aminobutyric Acid (GABA) which ordinarily inhibits various alerting neural pathways, thus leading to drowsiness, and finally it can cause secretion of dopamine to make you feel good. The net result of this is that fatigue is blunted while alertness is promoted, you feel more positive and you are more physically prepared for performance.
In doses ranging from as little as 32mg to as much as 600mg (for reference, a Red Bull contains 82mg), caffeine has been shown to augment cognitive performance by improving alertness, reducing reaction time and having a small impact on short-term memory. It can also improve mood at doses of 250mg.
Where caffeine really shines is in terms of exercise performance. Caffeine in doses of 3-6mg per kg of bodyweight can improve performance in exercise lasting up to 2 hours, meaning that this is not specific to any particular energy pathway, and it does so through a variety of ways including power output, rate of perceived exertion (how hard exercise feels at a given intensity is reduced), strength endurance and speed to a significant degree. There is no impact on maximal effort single repetitions, however. This effect is not mediated by habitual consumption, meaning that pre-workout caffeine intake is effective even in those who typically use caffeine in regular life. Of note is that supplemental pre-workout caffeine from caffeine anhydrous (an isolated form used in supplements) appears more effective than coffee and is easier to dose accurately.
A final reason cited for caffeine consumption is as an aid to fat loss, with caffeine being a key ingredient in many thermogenics. Research indicates that caffeine can increase metabolic rate by 3-11% in some trials, and in fact long-term research indicates that habitual caffeine intake is correlated with decreased bodyweight. This could be in relation to caffeine’s thermogenic properties or to its ability to help reduce appetite as indicated by the fact that supplementation generally reduces subsequent calorie intake when researched. A warning here, however, is that the appetite blunting effect of caffeine does not appear strong in habitual users meaning that those using caffeine all of the time may not get the benefits that others would.
Of course, due to caffeine’s impact on performance and fatigue resistance, it can be hypothesised on top of the above that caffeine intake before training during periods of caloric restriction may undo the performance detriments associated with low energy intakes, and therefore it may improve calorie output via this means. Finally, animal data indicates that NEAT may be increased by caffeine though this has not been shown in humans as yet.