According to a new U.S. military study, caffeine doesn’t give a mental boost after several days of insufficient sleep.

In the study, published online in the journal Sleep, researchers assessed 48 healthy volunteers who were restricted to five hours of sleep a night, for five consecutive days. Twice a day, at 8 a.m. and again at 12 p.m., participants were instructed to take either 200 mg of caffeine – the amount in a big cup of coffee – or a placebo. The study was double-blinded, meaning neither the experimenters nor the participants knew who was receiving which treatment.

The participants were given mental tests each hour that they were awake. For the first two days, those who took caffeine had faster reaction times compared to the placebo group, but not on the last three days of the experiment. Also, during the first few days, the caffeine group reported feeling happier than those who took the placebo.

“These results are important, because caffeine is a stimulant widely used to counteract performance decline following periods of restricted sleep,” said Tracy Jill Doty, the lead study author and research scientist at Walter Reed Army Institute of Research.

However, the study does not consider that sleep-deprived individuals may increase the amount of caffeine that they consume over time. "We do not know what would occur if more caffeine was taken," states Doty. "Increased caffeine dosage will increase negative side effects such as jitteriness, but we do not currently know if an increased dosage would prevent performance decline."

Caffeine can be a good stimulant for people, but also remember that adequate sleep is important. The U.S. Centers for Disease Control and Prevention advises adults to sleep seven to eight hours each night.

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