No you’re just mis-comprehending simple reality. Time alone is NOT cruel and unusual punishment, it’s just punishment, regular old every day not cruel not unusual and often RICHLY deserved. It’s not flying in the face of anything except the whining we constantly get out of prisoners that everything is cruel and unusual, everything of course except what they did to get there. Solitary used to be the default, it’s a good method of punishment, if we had the space it should be the default again. It helps prevent all the other problems of prison like rape and trading methods to churn out “better” criminals.
'Time' is not cruel and unusual. Solitary is. This article explores the effects in greater detail in a reader-friendly manner.
This is a less reader-friendly (but more footnoted and scholarly) scientific treatise on same.