Numerous scientific studies have investigated the effects of caffeine on sports performance. If you're interested in a little light reading, here are some of our favorites:

Four separate studies conclude that caffeine enhances endurance in a variety of sports, such as running, cross-country skiing, and cycling. These studies looked at caffeine naïve and habituated athletes, and at trained and untrained athletes. The performance effects were seen at intakes that resulted in caffeine urinary levels below the legal limits stipulated by the International Olympic Committee. These studies looked at caffeine doses from 2-9 mg/kg. The increased endurance from caffeine was more pronounced in well-trained athletes.

Dodd SL, Herb RA, Power SK. 1993. "Caffeine and exercise performance." An uptake. Sports Med.
Graham TE, Rush JWE, van Soeren MH. 1994. Caffeine and exercise: metabolism and performance. Can J Appl Physiology.
Spriet LL. 1995. "Caffeine and Performance." Int. J Sport Nutri.
Tarnopolsky MA, Hicks A, Cupido C, McComas AJ. 1992. "Caffeine and neuromuscular function in humans; No effects of tolerance." Physiologist of Caffeine in Exercise and Sports Science, 2000".

Caffeine ingestion appears to enhance prolonged endurance performance, as well as high-intensity, short-duration exercise. Compared with placebo, improvements in endurance times of 22%-23% have been reported after ingestion of caffeine doses of 3-9 mg/kg. Higher doses did not appear to have any additional benefit.
"of Caffeine in Exercise and Sports Science, 2000."

Caffeine may enhance performance in high-intensity, shorter-duration events as well, such as the 1,500-m and 5,000-m track events. Performance in these events, the authors postulate, would not appear to be limited by muscle glycogen depletion but possibly by other factors, including neural stimulation.
Graham TE, Spriet LL.1996. "Caffeine and exercise performance." Sports Sci Exch.
Spriet LL. 1995. "Caffeine and Performance." Int. J Sport Nutri.

Time to get lawyerly: We are NOT giving you medical advice here. This is intended purely as a summary of some of the cool scientific evidence regarding the effects of caffeine on sports performance. This is not intended as a dose guide. Caffeine is not a substitute for sleep. If you have any concerns about the effects of caffeine on you, please ask your doctor. We at VE₂ urge you to use caffeine responsibly.