We have long assumed that we understood the invigorating effect of caffeine. But based on a new study, it looks like habitual drinkers develop tolerance to the stimulating powers of coffee, just like they develop tolerance to dehydrating effects of caffeine. The real reason coffee addicts feel "woken up" by a cup of coffee is because they were feeling withdrawal symptoms (like all other addicts do!). Drinking coffee brings them back to normal, which of course feels like a major improvement from the distressed state they were in before.
So only first-time coffee drinkers get the real cognitive boost, while regular drinkers show improved performance simply by alleviating the addiction withdrawal. Very interesting perspective.
Here is a link to Science Daily article on this: Coffee consumption unrelated to alertness: Stimulating effects may be illusion, study finds.