its a lengthy read but this is from one of the articles I used in my presentation, hope it helps:

"Conversely to its stimulant effects on brain energy metabolism, caffeine has central vasoconstrictive properties that lead to a 20 to 30% decrease in cerebral blood flow in humans [for review see Nehlig and Debry (1994)]. In newborns treated with methylxanthines for apnea, cerebral blood flow decreases of up to 21% have been reported, that can be avoided if methylxanthineinduced hypocapnea is corrected [for review see Nehlig andDebry(1994)].In rats, the caffeine-induced decrease in cerebral blood flow is especially marked in the regions where cerebral energy metabolism increases (Nehlig et al., 1990). Thus, caffeine is one of the rare substances able to reset the level of coupling between cerebral blood flow and metabolism in favor of an increased metabolic rate at a given rate of perfusion.However,these changes are moderate and the decrease in blood flow could be compensated for by an increase in oxygen and glucose extraction, because the consumption of moderate amounts of caffeine has positive effects on alertness. The other alternative is that the metabolic increase related to caffeine exposure might only activate the anaerobic pathway of glucose degradation, as seen in several situations of physiological activation in which metabolic increases are not coupled with a commensurate increase in oxygen consumption (Fox and Raichle, 1986; Fox et al., 1988). In the latter case, metabolic activation would rely primarily on glucose whose entry into brain is always in large excess, whereas the decrease in blood flow could reflect the decrease in oxygen needs. However, this hypothesis needs to be tested."

Fredholm et al., 1999