Caffeine is a mild psychostimulant that is particularly effective in people suffering from sleep problems or from caffeine withdrawal. Caffeinated energy drinks - such as Red Bull - have been shown to improve energy and cognition but this could be down to a placebo effect. A study of 35 people in Chicago compared the effects of a caffeine-containing supplement with a placebo on people who had been kept awake from 5pm to 5am. The participants completed subjective effects questionnaires and performed computerized attention tasks. Compared to the placebo the caffeine supplements produced stimulant-like effects and significantly improved mood and reaction times on the tasks.
Childs, Emma and deWit, Harriet - Enhanced mood and psychomotor performance by a caffeine-containing energy capsule in fatigued individuals Experimental and Clinical Psychopharmacology February 2008, 16(1), 13-21