I've never been to prison and I hope I never will have to experience it.
But I would bet that prison has got to be pretty boring especially if you're in solitary confinement.
I have trouble staying inside my house during frigid weather so I would go stir crazy being locked up inside a building for a long time and never able to leave the prison grounds.
Most prisoners do get to do more than just sit around in a prison cell all day and some get to go on work programs which keep them busy.