This report by Susan Greene is a first-rate piece of work, and should be read in full. A key paragraph:
Among the misperceptions about solitary confinement is that it’s used only on the most violent inmates, and only for a few weeks or months. In fact, an estimated 80,000 Americans — many with no record of violence either inside or outside prison — are living in seclusion. They stay there for years, even decades. What this means, generally, is 23 hours a day in a cell the size of two queen-sized mattresses, with a single hour in an exercise cage, also alone. Some prisoners aren’t allowed visits or phone calls. Some have no TV or radio. Some never lay eyes on each other. And some go years without fresh air or sunlight.