More than 25 of the CIA's war-on-terror prisoners were subjected to sleep deprivation for as long as 11 days at a time during the administration of former president George Bush, according to The Los Angeles Times.

At one stage during the war on terror, the Central Intelligence Agency was allowed to keep prisoners awake for as long as 11 days, the Times reported, citing memoranda made public by the Justice department last month.

The limit was later reduced to just over a week, the report stated.

Sleep deprivation was one of the most important elements in the CIA's interrogation programme, seen as more effective than more violent techniques used to help break the will of suspects.

Within the CIA it was seen as having the advantage of eroding a prisoner's will without leaving lasting damage.

The technique is now prohibited by President Barack Obama's ban on harsh interrogation methods issued in January, although a task force is reviewing its use along with other interrogation methods, The Times said.

But details in the Justice Department memos released by Mr Obama suggest that the method, which involved suspects standing for days on end, dressed only in a nappy and shackled to the floor, was more controversial than previously known.

According to the memos, medical personnel were present to make sure prisoners weren't injured. But a 2007 Red Cross report on the CIA program said detainees' wrists and ankles bore scars from their shackles, the newspaper reported..

When detainees could no longer stand, they could be laid on the prison floor with their limbs "anchored to a far point on the floor in such a manner that the arms cannot be bent or used for balance or comfort," a memo dated May 10, 2005, said.

"The position is sufficiently uncomfortable to detainees to deprive them of unbroken sleep, while allowing their lower limbs to recover from the effects of standing," it said.

In the Red Cross report, prisoners said they were also subjected to loud music and repetitive noise.

"I was kept sitting on a chair, shackled by hands and feet for two to three weeks," said suspected Al Qaeda operative Abu Zubaydah, the first prisoner captured by the CIA, according to the Red Cross report. "If I started to fall asleep, a guard would come and spray water in my face."

In the Justice Department memos, sleep deprivation was described as part of a "baseline" phase of interrogation, categorized as less severe than other "corrective" or "coercive" methods.

"Waterboarding was obviously the most controversial," said a former senior U.S. government official who was briefed extensively on CIA interrogation operations. But "sleep deprivation is probably the most effective thing they had going."

The Justice Department memos also cited research that suggested sleep deprivation was not harmful.

"Experience with sleep deprivation shows that 'surprisingly, little seemed to go wrong with the subjects physically,' " said the May 10 memo.

But a British scientist whose name was one of those put on the studies said he had never been consulted by US officials about the study.

James Horne, director of the Sleep Research Centre at Loughborough University, said he didn't know how his work was being used until the memos were released.

"My response was shocked concern," Professor Horne told the LA Times. Just because the pain of sleep deprivation "can't be measured in terms of physical injury or appearance . . . does not mean that the mental anguish is not as bad," he said.

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