One prisoner said he hasn't been exposed to direct sunlight for 39 years. Another said he is forced to urinate on the floor of his cell because he is denied access to a needed catheter. Yet another inmate has a phrase to describe the intense anxiety, paranoia and auditory hallucinations he experiences as a result of being confined, alone, to a tiny cell: He calls it SHU syndrome.

SHU stands for security housing units, one-person cells that currently house some 4,000 prisoners in California. A coalition of advocacy organizations has petitioned the United Nations to intervene, saying the widespread use of solitary confinement violates international safeguards against torture.

Solitary confinement has an incredibly adverse impact on the prisoner's physical health and mental health, said Peter Schey, executive director of the Center for Human Rights and Constitutional Law and lead attorney for the more than 400 prisoners on whose behalf the petition was filed. It breaks their spirit, it results in a wide variety of mental disorders, in deterioration of their physical health.

In addition to breaching international agreements such as the U.N. Convention Against Torture, Schey said, the practice violates the U.S. Consitution's ban on cruel and unusual punishment. His hope is that the United Nations will accept jurisdiction over the petition and push the U.S. government on several fronts, including allowing Red Cross workers to interview the prisoners.

Thousands of inmates across California's prison systems went on a hunger strike this summer, seeking to draw attention to what they said were deplorable and inhumane conditions. Prisoners suspected of having gang ties can be held for years in cramped cells, their contact with other humans virtually nonexistent. They are kept there for a minimum of six years, although for some the isolation has spanned decades. Many complain of inadequate medical and mental health care. Suicidal behavior is common.

California officials defend the use of security housing units as a vital deterrent against prison gangs. A 2011 FBI report on gang activity found that prison gangs pose a serious threat to the safety of prison personnel and other prisoners, using intimidation and violence to consolidate power and maintain ties to gang members outside of prison.

An inmate who wants to rehabilitate himself cannot, California prisons undersecretary Scott Kernan testified during a legislative hearing in August. Not when he has an inmate, like the people we have in [isolation], telling him to go stab somebody or he will be killed.

California's Department of Corrections and Rehabilitation defends the policy in similar terms. A recent report stresses that security housing units are not designed nor intended as punishment for misbehavior, but rather designed to protect the public, staff and other offenders from violent inmates and gangs.

Advocates acknowledge the need to suppress prisoners who are endangering other inmates. But they say that the use of solitary confinement in California prisons, in its scope and duration, is wildly excessive.

No one who knows anything about prisoners would disagree that there are some prisoners who require physical separation from other prisoner so they don't hurt them, but that's a relatively small proportion of the people we have in solitary confinement in this country, said David Fathi, director of the American Civil Liberties Union's national prison project.

Others dispute California's claim that the practice isn't punitive. They say forced separation from others necessarily implies physical and emotional suffering.

Whether or not it is the prison system's intent to punish, these prisoners are in pain. Many of them are in anguish, said Craig Haney, a professor of psychology at the University of California, Santa Cruz, who has studied the effects of solitary confinement.

Isolation is punishment, Haney added. It's one of the reasons it's so often used by regimes that torture their prisoners.

The repercussions often endure long beyond a prisoner's time in solitary confinement, Haney said, leaving permanent psychological scars and making it difficult to reintegrate into human (even prison) society.

Many of them deteriorate over time so that when they are eventually released, whether into a mainline prison population or back into free society, they have lost many of the capacities they will need to function in a very social world, Haney said. For some people, we are forcing them to become the type of people who can live nowhere but in isolation.

California has proposed some reforms, including a Step Down program that would allow isolated prisoners to work their way back into the general inmate population by renouncing gang activity. But Schey dismissed the proposed measures as a joke, noting that under the Step Down initiative inmates would still be in solitary confinement for a minimum of four years.

It's like saying we're not going to choke you for six years, we're only going to choke you for four years, Schey said.

Security housing units became more prevalent as California's prison population surged in the 1980s and 1990s. The Supreme Court has ordered California to reduce its state prison numbers by 34,000 people over the next two years -- mostly by transferring prisoners to county facilities -- and Haney said many advocates have high hopes that solitary confinement will become less frequent as a result.

The system can back off from being worried about where they're going to pack in all the prisoners they have now and begin thinking about what kind of system do we really want to run in California, Haney said.

Schey said that the United Nations hasn't yet responded to the petition. But the issue has some resonance in the international community. In October, Juan Méndez, the United Nations' special rapporteur on torture, called on governments to ban the practice of isolating prisoners, calling it contrary to rehabilitation, the aim of the penitentiary system.

Much like the death penalty, solitary confinement is an issue on which the U.S. is seen as a real global outlier, said Fathi. There's no other country, certainly no other democratic country, that uses solitary confinement as routinely as the United States does.

Editor's note: An earlier version of this story incorrectly referred to security housing units as segregated housing units.