Picture MetLife Stadium, the New Jersey venue that hosted the Super Bowl earlier this month. It seats 82,556 people in total, making it the largest stadium in the NFL.

Imagine the crowd it takes to fill that enormous stadium. That, give or take a thousand, is the number of men and women held in solitary confinement in prisons across the U.S.

Although the practice has been largely discontinued in most countries, it's become increasingly routine over the past few decades within the American prison system. Once employed largely as a short-term punishment, it's now regularly used as way of disciplining prisoners indefinitely, isolating them during ongoing investigations, coercing them into cooperating with interrogations and even separating them from perceived threats within the prison population at their request.

As the number of prisoners in solitary has exploded, psychologists and neuroscientists have attempted to understand the ways in which a complete lack of human contact changes us over the long term. According to a panel of scientists that recently spoke at the American Association for the Advancement of Science's annual meeting in Chicago, research tells us that solitary is both ineffective as a rehabilitation technique and indelibly harmful to the mental health of those detained.

"The United States, in many ways, is an outlier in the world," said Craig Haney, a psychologist at UC Santa Cruz who's spent the last few decades studying the mental effects of the prison system, especially solitary confinement. "We really are the only country that resorts regularly, and on a long-term basis, to this form of punitive confinement. Ironically, we spend very little time analyzing the effects of it."

Exact numbers are hard to come by, but based on a wide swath of censuses, it's estimated that between 80,000 and 81,000 prisoners are in some form of solitary confinement nationwide. In contrast to stereotypes of isolated prisoners as the most dangerous criminals, Haney estimates that a third of isolated prisoners are mentally ill, and a disproportionate number are minorities, partly because alleged gang membership is grounds for placing a prisoner in solitary indefinitely.

The physical details of an isolated prisoner's daily experience are worth examining. "Prisoners live in their cells, 80 square feet on average—a bit bigger than a king-sized bed. In this environment, you sleep, you eat, you defecate, you live all of your life," Haney said. Most prisoners spent at least 23 hours per day in this environment, devoid of stimuli (some are allowed in a yard or indoor area for an hour or less daily), and are denied physical contact on visits from friends and family, so they may go years or decades without touching another human, apart from when they're placed in physical restraints by guards.

This sort of existence takes a clear toll on prisoners, according to surveys and interviews Haney and colleagues have conducted with about 500 of those in isolation from four different states. Their work indicates that most of these prisoners suffer from severe psychological stress that begins when they're put in isolation and doesn't subside over time.

A majority of those surveyed experienced symptoms such as dizziness, heart palpitations, chronic depression, while 41 percent reported hallucinations, and 27 percent had suicidal thoughts—all levels significantly higher than those of the overall prison populations. An unrelated study published last week found that isolated inmates are seven times more likely to hurt or kill themselves than inmates at large.

These effects, Haney says, don't only show how isolation harms inmates—they tell us that it achieves the opposite of the supposed goal of rehabilitating them for re-entry into society. "We are all social beings, and people who are in environments that deny the opportunity to interact in meaningful ways with others begin to lose a sense of self, of their own identity," he said. "They begin to withdraw from the little amount of social contact that they are allowed to have, because social stimulation, over time, becomes anxiety-arousing."

Huda Akil, a neuroscientist at the University of Michigan, is interested in the neurological impacts of isolation, but is limited by the fact that no U.S. prison is willing to allow its otherwise isolated prisoners to take part in research. Instead, she and others must rely on more basic findings about how stimulation and social interaction affect the brain, and infer the potential impacts of isolation from that.

For one, there's the fact that a large amount of brain activity is driven by circadian rhythms, which are in turn set by exposure to the Sun. Autopsies on people who have committed suicide after suffering from depression have shown that, in their brains, gene expression is significantly less aligned with circadian rhythms; other research has shown that restricting exposure to sunlight (and thereby interfering with circadian rhythms) increases the prevalence of depression. Thus, if inmates are already prone to depression, solitary probably makes them more so, she says. The proper functioning of the brain depends on daily sun exposure, potentially explaining some of the symptoms experienced by prisoners in isolation, many of whom rarely see the Sun.

There are also troubling neurological implications of long-term isolation that stem from the fact that brain architecture can change over time. The hippocampus, in particular, has been found to dramatically shrink in the brains of people who are depressed or stressed for extended periods, a concern because it's crucially involved in memory, geographic orientation, cognition and decision-making. No one has performed an autopsy on a person who lived in isolation for decades, suffering from depression the whole time, but Akil believes that in keeping inmates in full isolation, authorities are "ruining a very critical component of the brain that's sensitive to stress."

Apart from scientists, the Chicago panel featured activist Robert King, who spent 29 years isolated in six-by-nine-foot cell in a Louisiana prison before his murder conviction was overturned in 2001. Although he endured solitary confinement more successfully than most, he says—he maintained a hopeful attitude and never considering hurting himself—he experienced unmistakable physiological changes.

Most dramatically, King gets has difficulty navigating open spaces. "I lost the ability to meet with a broader terrain. I had become acclimated to shorter distances," he said, attributing this change to the shrinkage of his hippocampus, "I cannot, even to this day, acclimate myself to broader distance. My geography is really shot." His eyesight also deteriorated to the point where he was nearly blind, though it's gradually improved since he was released.

It's impossible to say how isolated prisoners fare as a whole fare compared to King, because there's no systematic collection of data on their well-being in the U.S. prison system. But the researchers argue that just these hints of the damage wrought by solitary confinement—and the way it seems to make prisoners less-equipped to re-enter society after their sentence—indicate that it falls within a category of discipline banned by the eight amendment: cruel and unusual punishment. "It seems to me that it is time for us to have a serious discussion about the wisdom and humanity of this policy in the United States," Haney said.