There are studies from the last twenty years or so that manic and hypomanic episodes cause roughly the same damage as, oh, a high school football concussion. The football concussion will heal in 2-3 months, while the damage from mania takes a couple of yep ears to heal. Another episode within that time interrupts the healing process.

I forgot to grab a link before I came in here, but it's fairly easy to find information by searching "mania and cognitive function" or "mania and executive function." If I find the article I'm thinking of I'll come back and post the link.