For someone who was supposedly dead, Michael looks surprisingly none the worse for wear and amazingly spry in Eps #928 and #929.
I think...
He had to go through a dying process or something similar, so that Dr. Hoffman could observe this objectively in examinations, but of course never actually died. He was terrified beforehand, almost as if it were real death, so it probably wasn't pleasant. He was persuaded to go through with it, because he was told it was necessary, but I think Michael was capable of willing himself out of it, though, and rejuvenating any time he chose. He can change form, we do know that much.