He is locked up at the high-security prison in Florence, Colorado, known as Supermax. The lights are always on. Guards who slip him food through a slot in his cell door usually ignore him. A few times a week, he is permitted to exercise in the recreation room -- alone. Visits with his family and his lawyers are conducted through Plexiglas.

Silverstein's isolation is the result of an unusual no-human-contact order issued by a judge in 1983, after he murdered a guard at the federal prison in Marion, Illinois. Marion was known at the time as the most rigorous confinement in the federal prison system.

Silverstein has referred to his solitary existence as "a slow, constant peeling of the skin."

His attorneys, who are affiliated with the University of Denver, filed a lawsuit against the U.S. Bureau of Prisons in 2007, alleging that the such prison conditions violate the cruel and unusual punishment clause of the Eighth Amendment. The lawsuit, filed in the federal district court of Colorado, is awaiting trial.

At Supermax, Silverstein, 58, practices yoga and meditates in his cell. He might catch an episode of "the Sopranos" or a reality show on the black-and-white TV in his cell. It's his only way to see the outside world.

Recently, he's learned to crochet, and he fills much of his time writing letters.

He has two Web sites being run by advocates and family friends: www.tommysilverstein.bravehost.com and tommysilverstein.blogspot.com.

One of the sites includes examples of Silverstein's prison artwork and writing, providing a glimpse into a life of isolation. Blue-toned drawings show hands trapped behind bars. Black-and-white ones starkly show cage-like conditions.

"It's almost more humane to kill someone immediately than it is to intentionally bury a man alive," he wrote in a 2008 letter to a friend.

Psychologists who have studied the effects of solitary confinement find that the lack of social interaction can cause severe anxiety and depression. Some inmates in solitary have committed suicide.

Eyes like a 'wild person'

His sister, Sydney McMurray, came from California to Colorado to visit him a year ago. She said he seemed exhausted, with eyes like a "wild person."

"Relax, Tommy," she told him through the thick Plexiglas. "It's OK."

She hopes Silverstein will someday find peace. She says she draws solace from the legend of Bodhi Dharma, a monk who, according to some accounts, retreated to a cave to meditate alone for nine years.

"They say you come back crazy or come back wise," McMurray says. "Tommy's always been a strong guy, and I think this has made him wiser."

McMurray says she believes her brother has changed, and she knows he has suffered from reading his letters.

CNN attempted to contact Silverstein by mail, but he did not respond. Access to him is limited. To date, only one journalist has interviewed him, in 1988.

The U.S. Bureau of Prisons says "solitary confinement," a term widely used by prison advocacy groups and attorneys, doesn't exist in federal prisons. Instead, authorities call the isolated cells where inmates are housed the SHU: special housing units.

U.S. Bureau of Prisons spokesman Edmond Ross estimates that on any given day, 11,150 of the 200,000 federal inmates are kept in special housing units. The reasons for confinement vary from protecting a witness to disciplinary measures.

In the U.S., the practice dates to the 1820s, when Quakers believed that complete isolation forced prisoners to repent. But in 1890, the Supreme Court ruled that confinement had disastrous mental effects, causing inmates to go insane and become violent.

'Worse than death row'

"At times, it was worse than death row. At least death row had privileges," Robert Hillary King said.

King is one of the Angola 3, a trio of Louisiana inmates who were locked away in solitary confinement in the 1970s. Two of the three men were accused of stabbing a corrections officer.

King, who was never charged with the killing, won his freedom in 2001 after a series of appeals. He had spent 29 years in solitary confinement in the state prison system. The other two members of the Angola 3 are still being held in isolation.

For Silverstein, his adult life behind bars began in 1978. He was a 26-year-old Californian with a heroin addiction when he was handed a 15-year sentence for robberies he committed with his father.

Two years later, Silverstein was convicted of murder for the first time: the slaying of a fellow inmate in a Kansas prison.

He was moved to a maximum-security prison in Marion, Illinois. His murder conviction was overturned while he was there.