Soldiers who have too little sleep perform worse than if they were drunk, potentially affecting their ability to make split-second decisions, experts have warned.
Some soldiers in Iraq have been on the move for weeks
Just one week of sleep deprivation could affect soldiers' abilities so they are unable to remember where troops on their own side are positioned, potentially leading to "friendly fire" incidents.
A team from the US Army Research Institute of Environmental Medicine followed Navy Seals and Army Rangers.
They carried out cognitive tests before and during the exercise to see if their performance was affected by lack of sleep.
New Scientist magazine reports that compared with performance tests carried out before the exercise, soldiers who trained for several days in a row suffered a significant drop in their ability to perform combat tasks.
Their performance was actually worse than if they were legally drunk
Dr Harris Lieberman, US Army Research Institute
Navy Seals, who slept for just one out of 73 hours of duty, performed the worst.
Younger soldiers also showed slower reaction times, reduced vigilance and more problems remembering key details than experienced colleagues.
In one test of the ability to make quick decisions, their number of errors rose from one or two to over 15.
But even the more experienced soldiers performed worse in the tests.
Dr Harris Lieberman, who led the research, said: "Their performance was actually worse than if they were legally drunk."
He said such impairment could affect a soldier's ability to be alert enough to spot a potential target, his reasoning to decide if it was appropriate to fire his weapon, and his short-term memory to remember where friendly forces were located.
But Gregor Belenky, director of neuropsychiatry at the Walter Reed Army Institute of Research said an individual's performance in any situation was affected by sleep deprivation.
He said: "I don't think combat is any worse than flying an airplane, driving a truck or working in an emergency room."
Dr Belenky said soldiers' reactions in combat situations were likely to be quickened by adrenaline, and it was in more mundane situations where fatigue affected alertness.
Neil Stanley, of the British Sleep Society told BBC News Online: "Essentially, if you go 16 or 17 hours without sleep, your performance deteriorates as though you've had a legal dose of alcohol."
He said performance at night could also be affected by lack of sleep.
"So you've got people fighting a war at night most of the time, they are sleeping in uncomfortable conditions in the middle of the day.
"They are probably not going to get a good night's sleep. And it's their judgement which will be impaired.
"You see a target and you shoot it, even if it's a soldier from your own side."
Dr Dirk-Jan Dijk of the Surrey Sleep Centre, added: "Whether the 'excitement' of war will offset any of the detrimental effect of sleep loss on performance remains unclear.
"Nevertheless, performance decrements related to sleep loss and having to be awake at 'wrong' phase of the circadian cycle is a real concern.
"A comparison with the effects of alcohol is one way to convince the public and the authorities of the severity of these decrements."