Extract from Anne Applebaum:

Gulag, A History of the Soviet Camps (Penguin 2004)

Yet although this sort of beating was technically forbidden after 1939, the change of policy did not necessarily make the investigation process more humane.   Throughout the 1920s, 1930s and 1940s many hundreds of thousands of prisoners were tormented not with actual beatings or physical attacks, but with psychological torture of the sort Abakumov alludes to in his letter to Stalin.

Those who remained stubborn and refused to confess could, for example, be slowly deprived of creature comforts, first walks, then packages or books, then food.   They could be placed in a specially harsh punishment cell, very hot or very cold, as was the memoirist Hava Volovich, who was also being deprived of sleep by her interrogator at the time:  'I will never forget the first experience of prison cold.    I can't describe it; I'm not capable of it.   I was pulled one way by sleep, the other by cold.   I would jump up and run round the cell, falling asleep on my feet, then collapse on the bed again, where the cold would soon force me up.'

Others were confronted with 'witnesses', as was Evgeniya Ginzburg, who watched as her childhood friend Nalya 'recited like a parrot', accusing her of membership of the Trotskyite underground.   Still others were threatened with harm to family members, or were placed, after long periods of isolation, in cells with informers, to whom they were only too glad to open their hearts [...]

There were also forms of physical torture less crude than beatings, and these were used regularly from the 1920s onwards.   Tchernavin was early on given 'the standing test' -- prisoners were told to stand, facing the wall, without moving -- albeit briefly.  Some of his other cell mates suffered worse:

One, Engraver P., over fifty years of age and heavily built, had stood for six and a half days.   He was not given food or drink and was not allowed to sleep; he was taken to the toilet only once a day.   But he did not 'confess'.  After this ordeal he could not walk back to the cell and the guard had to drag him up the stairs...  Another, Artisan B., about thirty-five years old, what had one leg amputated above the knee and replaced by an artificial one, had stood for four days and had not 'confessed'.

Most commonly, however, prisoners were simply deprived of sleep:   this deceptively simple form of torture -- which seemed to require no special advance approval -- was known to prisoners as being put 'on the conveyor', and it could last for many days, or even weeks.   The method was simple:   prisoners were interrogated all night, and afterwards forbidden from sleeping during the day.   They were constantly awoken by guards, and threatened with punishment cells or worse if they failed to stay awake.   One of the best accounts of the conveyor, and of its physical effects, is that given by the American Gulag inmate Alexander Dolgun.   During his first month in Lefortovo, he was virtually deprived of any sleep at all, allowed an hour a day or less: 'Looking back it seems that an hour is too much, it may have been not more than a few minutes some nights.'   As a result, his brain began to play tricks on him:

There would be periods when I suddenly knew that I had no recollection of what had happened in the last few minutes.   Drop-outs in my mind.   Total erasures....
Then, of course, later on, I began to experiment with sleeping upright, to see if my body could learn to hold itself erect.   I thought that if that would work I might escape detection in the cells for a few minutes at a time, because the guard at the peep-hole would not think I was asleep if I was sitting upright.
And so it would go on, snatching ten minutes here, half an hour there, occasionally a little longer if Sidorov called it quite before six in the morning and the guards left me alone till the wake-up call.   But it was too little.   Too late.  I could feel myself slipping, getting looser and less disciplined every day.   I dreaded going crazy almost worse -- no, really worse -- than dying...

Dolgun did not confess for many months, a fact that provided him with something to be proud of throughout the rest of his imprisonment.   Yet when, many months later, he was called back to Moscow from his camp in Dzhezkazgan and beaten up again, he did sign a confession, thinking, 'What the hell.   They've got me anyway.   Why didn't I do it a long time ago, and avoid all that pain?'

Why not indeed?   It was a question many others asked themselves, with varying answers [...]

Neverthe less, the contents of Sgovio's file clearly illustrate that subsequent decisions -- about early release, amnesty, and so on -- were indeed taken on the basis of what was in a prisoner's file, including confession.   If you had managed to hold out, in other words, you did stand a very, very slim chance of having your sentence reversed.   Right up through the 1950s, all of these judicial procedures, however surreal, were taken seriously.

In the end, the interrogation's greatest importance was the psychological mark it left on prisoners.    Even before they were subjected to the long transports east, even before they arrived in their first camps, they had been at some level 'prepared' for their new lives as slave labourers.   They already knew that they had no ordinary human rights, no right to a fair trial or even a fair hearing.   They already knew that that NKVD's power was absolute, and that the state could dispose of them as it wished.   If they had confessed to a crime they had not committed, they already thought less of themselves.   But even if they had not, they had been robbed of all semblance of hope, of any belief that the mistake of their arrest would soon be reversed.

Anne Applebaum, Gulag: A History pp 144 -147