WASHINGTON — US service members in Afghanistan who consumed three or more caffeine-charged energy drinks a day were prone to sleepiness and dozing off while on guard duty, according to a report.

Forty-five percent of those in Afghan­ combat areas in 2010 were daily users of energy drinks, some of which have the caffeine of one to three cups of coffee, the study said. The finding was published Thursday in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly report.