Drinking a moderate amount of caffeine might step up your smarts, but that benefit decreases the more you drink, according to a new study.

Energy drinks such as Red Bull, Burn and Monster are popular among high school- and college-aged people to help them stay awake, study and cut the intoxicating and drowsy effects of alcohol.

Researchers found that students given the lowest dose of caffeine used in the study — equivalent to half a can of Red Bull – had the best responses to a reaction-time test, which required them to respond to targets on a computer screen. But those that drank more caffeine had slower reaction times.

Participants were also asked how stimulated and mentally fatigued they felt after the drinks. The students who were given the one can of Red Bull reported feeling more stimulated and less tired than participants given less caffeine, though they did not perform as well on the test.

"This finding is of interest given that energy drinks are frequently mixed with alcohol, and the acute effects of alcohol impair response inhibition," study researcher Cecile A. Marczinski, of Northern Kentucky University, said in a statement.

"Since regulation of energy drinks is lax in the United States in regard to content labeling and possible health warnings, especially mixed with alcohol, having a better understanding of the acute subjective and objective effects of these beverages is warranted," Marczinski said.

The study was based on the reaction times of 80 college students, ages 18 to 40. In order to test the effects of caffeine, researchers gave the participants either the energy drink Red Bull, the non-caffeinated soda Squirt (which looks and tastes like Red Bull), Squirt with 1.8 ml/kg caffeine added (equivalent to the caffeine in half a can of Red Bull), Squirt with 3.6 ml/kg caffeine added (equivalent to that of a can of Red Bull) or Squirt with 5.6 ml/kg caffeine added (equivalent to that of a can-and-a-half of Red Bull).

The study was published in the December issue of the journal Experimental and Clinical Psychopharmacology.

Pass it on: Caffeine drinks might seem like a good way to get a quick attention boost, but drinking too much can harm your cognitive abilities.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.