However, none of the prisoners have been convicted of a crime.
Mental health issues
Jennifer Daskal, senior counter-terrorism counsel at Human Rights Watch (HRW), said: "[They] are being warehoused in conditions that are in many ways harsher than those reserved for the most dangerous, convicted criminals in the United States."
Around 185 prisoners held in maximum security cells
They spend 22 hours alone in their small cells
The rooms have no natural light or fresh air
Prisoners are given no education opportunities
Their two hours of "recreation" time is spent in single-cell cages, sometimes in the middle of the night
Source: Human Rights Watch
"Security measures don't justify locking people in windowless cells ... with almost no opportunity for human interaction, physical exercise or mental stimulation."
The 54-page report by HRW investigators said several prisoners were suffering from mental health issues such as depression and anxiety, while some even reported seeing visions and hearing voices.
Unlike those in US "supermax" prisons, none of the Guantanamo prisoners being held in "harsh conditions" have been allowed visits by family members.
The Pentagon has in the past promised to allow prisoners to make phone calls home, but the report said less than a quarter have been given that opportunity.
Some of the nearly 200 people held in the maximum security environment have even been cleared for release or transfer, HRW said.
"It is unwise and short-sighted to house them in conditions that likely have damaging psychological effects and will only breed hatred and resentment of the United States," Daskal said.
Military officials have said improvements will be made at the facility, including increased recreation time and regular opportunities for prisoners to interact, but no schedule for implementation has yet been announced.
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