Solitary confinement in California prisons
10 December 2013 Last updated at 21:05 GMT
Tens of thousands of prisoners in California are confined on their own in a small "security housing unit" cells for an average of 22 hours a day. They get 10 hours a week of exercise in a small individual yard.
Their time in isolation is overwhelmingly counted in months or years, rather than days or weeks. The prisoners fear the effects on their mental health - and their capacity for eventual rehabilitation.
"You put a dog in a cage away from people, and you release that dog, he's going to bite," one inmate said.
The UN uses the word "torture" to describe solitary confinement of more than 15 days. Now, the courts are being asked to step in and force the state's prison service to change.
The BBC's Alastair Leithead visits a prison and talks to a guard and some inmates about conditions there.