That's a question to which the United Nations may soon have an official answer. The UN's top anti-torture envoy has begun an investigation into whether the detainment conditions of Pfc. Bradley Manning, who is charged with leaking classified materials to WikiLeaks, qualify as torture or mistreatment. The conditions include prolonged solitary confinement and the prevention of exercise.
I've argued before that the line between moral and immoral treatment isn't only set by the degree of discomfort or pain, but also and much more clearly by the affect that treatment has on the will. The line is completely crossed when the will of the one detained ceased to be motivated and begins to be coerced and controlled. The systematic infliction of minimal pain can result in a broken will just as surely as the sudden infliction of maximum pain. Both of these can be torture.
When assessing Manning's treatment, the UN envoy will need to determine more than just the level of discomfort Manning has experienced while in detainment, but also the long-term affects this treatment has had and continues to have on his will--his ability to make free, rational decisions.