A team of researchers in the US have found that caffeine use does not improve alertness or performance after three nights of five hours sleep or less. The study was conducted by scientists at the Walter Reed Army Institute of Research. A group of 48 healthy individuals had their sleep restricted to five hours of time in bed for a total of five days. Participants were administered either 200 milligrams of caffeine or a placebo twice daily and were subjected to a range of tests to measure their cognitive functions. Results show that relative to placebo, caffeine significantly improved Psychomotor Vigilance Task performance during the first two days, but not the last three days of sleep restriction. In fact, in the last three days the caffeine recipients also suffered from higher levels of annoyance than the placebo group.