It has actually been studied and proven that solitary confinement is extremely cruel, and dangerous. It was banned in the sixties because everyone they put in there, would end up going insane. Worse yet, solitary is more commonly used in our prison system than you may think. It's not just reserved for dangerous criminals, and is pretty much just issued to people guards don't want to deal with, this includes LGBT, mentally ill, or just people who do break small rules. Our prison system only recently brought it back, and it's been destroying minds ever since. It should be banned and never brought back.
Criminals obviously do have time to reflect on the things they have done while in prison, but being in solitary confinement does not allow you to see beyond the walls you inhabit. Much like being hungry and not being able to see beyond your next meal, solitary confinement makes you think about your current situation and suffering, and what you can do now, not what happened before. It is an inhumane, cruel punishment that solves nothing and further deteriorates and inmates' mental condition.
Solitary confinement, though used as a punishment for minor or major grievances, does not appear to help the situation as it is mentally anguishing. Though it may take the offender out of the situation for a time, in the end it does help to alleviate the behaviour, even if the prisoner never wishes to go there again.
Yes, solitary confinement is essentially inhumane because humans are social beings and require space to be able to move freely. Solitary confinement not only confines an individual's body but also places his/her mind in a box. Although prisoners are incarcerated for committing crimes, solitary confinement can further destabilize a person's psyche and drive them further into depression, madness etc.
With the increasing attention being given to the unjust system of incarceration in the United States along with studies that show prolonged solitary confinement leads to chronic mental illnesses such as PTSD it is time the United States does away with the practice once and for all. It constitutes a form of cruel and unusual punishment.
Solitary confinement, in my opinion, is a worse punishment than death. It is not used for an offenders benefit nor should it require any involvement in the welfare of the prisoner aside from providing food and shelter. It will provide a means for the offender to spend the rest of his or her life contemplating their actions and keep them ultimately removed from all society. As long as this punitive measure is reserved for the most heinous of crimes, any adverse effect it may have is inconsequential as it will be applied to individuals that have no chance of rehabilitation.