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Troops are losing sleep on the battlefield and energy drinks could be to blame.
A new study of nearly 1,000 combat troops deployed to Afghanistan in 2010 showed that those who drank three or more energy drinks a day, about 14 percent of the group, were more likely to get less than four hours of sleep a night and more likely to say they suffered poor sleep or nodded off during briefings or guard duty.
But whether troops are slamming energy drinks to stay awake, or the drinks actually are causing their sleepless nights, is unknown.
According to the study, conducted by the Walter Reed Army Institute of Research, 44.9 percent of the 988 Army and Marine combat platoon members surveyed said they consumed at least one energy drink a day much higher than consumption in the U.S. teen and young adult population overall, with a rate of 6 percent.
And a smaller group, 13.9 percent, said they drank three or more energy drinks, including energy shots, per day. Of that group, 38 percent said they got between three and four hours of sleep a day.
Fighting troops tend to get less sleep than their garrison counterparts by the very nature of their jobs. But getting fewer than four hours of shut-eye a night is low even for a combat environment, the study authors noted.
About half of the service members polled said they got five hours of sleep or fewer per day.
Previous studies have shown that drinking 200 milligrams of caffeine can improve cognitive performance, but in large quantities, caffeine may have negative effects, including difficulty with sleep. Sleeplessness and disrupted sleep also are known to affect work productivity.
It also could contribute to mental health problems and poor overall health.
"This study suggests that high levels of energy drink consumption might indirectly impair performance in a military setting," they wrote. The results are "similar to results found in a civilian study in which caffeine use caused an increase in nocturnal worry and sleeplessness."
Caffeine doses raise questions
Energy drinks are popular among U.S. troops because they are easily carried and consumed and provide a much-needed pick-me-up during long patrols or watches.
But loose regulations on the energy drink and supplement industry make it difficult to gauge exactly how much caffeine people get when they reach for a liquid boost.
Many energy drink makers don't list the caffeine content on their beverages, and some list inaccurate amounts: A Consumer Reports study published in October showed five of 16 products that listed their caffeine levels actually contained caffeine at levels 20 percent or more above the amount listed.
The best-selling Monster Energy is among those that don't list their caffeine levels because it is not required by law.
Each 24-ounce Monster contains 240 mg of caffeine, according to Consumer Reports, the same amount as an entire six-pack of Coke.
A single 5-Hour Energy has between 207 and 215 milligrams of caffeine, according to tests performed by ConsumerLabs.com and Consumer Reports.
The FDA is investigating five deaths that occurred after energy drink consumption, including that of a 14-year-old Maryland girl who died in December after drinking two 24-ounce Monster Energy drinks.
"[Monster] is unaware of any fatality anywhere that has been caused by its products," the beverage company said in a statement.
Caffeine intoxication is known to cause anxiety, restlessness and heart palpitations.
The authors caution against concluding that energy drinks cause troops' restless nights. They said more research is needed to determine whether energy drinks are the problem or troops simply reach for them because they already have sleep issues.
"Published studies suggest a cyclical combination," they wrote.
The military services should warn troops about the negative impact that drinking too much caffeine can have on their health, the researchers wrote.
Troops especially should be "encouraged to moderate their energy drink consumption in combat environments."