A variety of physiological and psychological effects have been attributed to energy drinks and their ingredients. Two studies reported significant improvements in mental and cognitive performances as well as increased subjective alertness. Excess consumption of energy drinks may induce mild to moderate euphoria primarily caused by stimulant properties of caffeine and may also induce agitation, anxiety, irritability and insomnia. During repeated cycling tests in young healthy adults an energy drink significantly increased upper body muscle endurance.
It has been suggested that reversal of caffeine withdrawal is a major component of the effects of caffeine on mood and performance.