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There are several 'camps' at Guantanamo in which conditions are extreme isolation. At one of those, at least, there is enough noise to prevent sleep. Damage from extreme isolation and sleep deprivation takes less than 10 days to start, no more than 30 to have permanent effects.
You want 6 months? No thanks. Move the prisoners first if placement is your only problem. You have no right to ask that someone undergo 6 more months of harsh, torturous incarceration just because you can't get your bureaucracy to move any faster. And if there are no takers among those other countries, follow Judge Urbina and Judge Leon's orders, and release them in the United States.