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I've seen a fair few studies over the while that suggest caffeine increases arousal in the short term and that for some tasks, this will also increase performance (there's a few citations here).

However, as with most drugs, habituation leads to withdrawal effects and adjustment, so that in some senses once a person gets used to caffeine, having a regular dose of caffeine may just bring a person to something like a normal level of functioning.

Thus, in a general sense, for someone used to consuming a certain level of caffeine, does having that regular quantity increase arousal or performance over and above the level that would be obtained at that moment had the person never got in the habit of consuming caffeine in the first place?