Another interrogation method was the stoika. It consisted of standing a prisoner against a wall on tiptoe and making him hold that position for several hours. A day or two of this was said to be enough to break almost anyone.
When there was time, the basic NKVD method for obtaining confessions and breaking the accused man was the "conveyor" -- continual interrogation by relays of police for hours and days on end. As with many phenomena of the Stalin period, it has the advantage that it could not easily be condemned by any simple principle. Clearly, it amounted to unfair pressure after a certain time and to actual physical torture later still, but when? No absolutely precise answer could ever be given.
But at any rate, after even twelve hours, it is extremely uncomfortable. After a day, it becomes very hard. And after two or three days, the victim is actually physically poisoned by fatigue. It was "as painful as any torture." In fact, we are told, though some prisoners had been known to resist torture, it was almost unheard of for the conveyor not to succeed if kept up long enough. One week is reported as enough to break almost anybody....
There is nothing new about the conveyor method. It was used on witches in Scotland. The philosopher Campanella, who withstood all other tortures in the sixteenth century, succumbed to lack of sleep. Hallucinations occur. Flies buzz about. Smoke seems to rise before the prisoner's eyes, and so on.
-- Robert Conquest, The Great Terror: A Reassessment.
Sgt. Ben Allbright knows something about sleep deprivation. During the first six months of the Iraq war, his job was to guard prisoners in Al Qaim, a makeshift base near the Syrian border where U.S. troops conducted aggressive hunts for Saddam Hussein's weapons of mass destruction. Every day, soldiers hauled in about 20 new detainees for questioning by military interrogators or CIA operatives. To "get them out of their comfort zone" before the interrogation, Allbright told Newsweek recently, the drill was to keep the Iraqis awake for 24 hours or longer. The guards would stand the prisoners up in old shipping containers, their hands tied behind their backs and their faces blindfolded and facing the wall. "After a while you'd see their knees buckling, or you'd see one of them lean forward and put his forehead against the wall to try to sleep." When that happened, Allbright would holler into the container or bang on the metal side with a stone or a club.
... Human Rights Watch, which interviewed the lawyers of 20 detainees held in a secret [CIA] prison in Afghanistan as late as 2004, says some of them were sleep-deprived for up to two weeks, though sometimes allowed short naps. The facility, known among detainees as "the dark prison," was among the harshest run by the United States since the start of the war on terror.
-- Dan Ephron, "Singing for Your Sleep: President Bush Aimed to Clarify Interrogation Rules: So Is It Legal -- or Effective -- to Deny Prisoners Rest?", Newsweek, Oct. 30, 2006.