A few years ago the well known exercise physiologist, David Costill, published the results of his research on caffeine and the physical performance of long distance runners. This research has resulted in the faddish use of caffeine among some and a generally positive impression of caffeine that is not completely warranted. While caffeine may still be a fad among runners it has a long history of use among European cyclists. Although touted as an almost miracle performance enhancer make no mistake about it, caffeine is not for everybody.
Caffeine is a powerful central nervous system stimulant whose effects diminish with increased usage.
Larger and larger doses can become necessary to avoid withdrawal symptoms and sustain stimulation until you need a cup just to function normally. To avoid this habituation, drink caffeine only during or before races and training. Most road cyclists limit their caffeine consumption to a bottle of coke diluted with water during training rides and the last half of longer races or a cup of drip coffee a half an hour before shorter races. The exact mechanisms of caffeine's diminishing effects are not well known but are probably are due to reduced levels of circulating free fatty acids and CNS neurotransmitters. Caffeine affects different people differently.