Caffeine affects our body by exerting a direct effect on the part of central nervous system that affects perception of effort, arousal and motor unit recruitment, which in turn allows individual to produce an increased work output.

Recent studies showed that the dosage of 9 milligrams of caffeine per kilogram of body weight, helped trained runners to increase race-pace endurance by 44% as well as cycling endurance by 51%

The amount of caffeine that is needed to produce any effect varies from person to person depending on both body sizes as well the degree of tolerance to caffeine. Research shows that it takes less than an hour for the effects of caffeine to begin affecting the body, while the mild dose of caffeine wears off in approximately 3 to 4 hours. Contradictory to the popular belief caffeine does not eliminate the need for sleep but only temporarily reduces the sensation of being tired.

Individuals who consume caffeine on the regular basis tend to slowly adapt to the continual presence of the drug by substantially increasing the number of adenosine receptors in the central nervous system. As a result the stimulatory effects of caffeine are substantially reduced, a phenomenon known as a tolerance adaptation. The increased tolerance to caffeine tend to develop quite quickly, with complete tolerance to sleep disruption effects of caffeine developing just after consumption of 400 mg of caffeine 3 times a day for 7 days.

Furthermore, the adaptive nature of caffeine will make withdrawal symptoms unpleasant both in philological and physiological sense in tolerant users, which include headaches, nausea, fatigue, drowsiness as well anxiety and irritability.

In large amounts and especially over extended periods of time, caffeine can lead to a condition as caffeinism. Caffeinism usually combines caffeine dependency with a wide range of unpleasant physical and mental conditions which include nervousness irritability, anxiety, tremulousness, muscle twitching, insomnia, headaches, respiratory alkalosis and heart palpapitations. Moreover, since caffeine increases the production of stomach acid, high usage over time can lead to peptic ulcers, erosive esophagitis and reflux disease.  Other side effects of caffeine overuse include: dizziness, tachycardia, blurred vision, drowsiness, dry mouth, flushed dry skin, diuresis, loss of appetite, nausea and stomachaches.