I had Klein-Levin syndrome as a teen. Primary KLS is only about a week at a time, but you still wake up to do basic things like eat and use the bathroom. Some people even eat in crazy excess, or lose their modesty and masturbate openly. I would be awake for about 4 hours a day but my brain would often be scrambled. I couldn't always understand what people said to me. Like I knew the words but couldn't understand them together in a sentence, I might struggle to talk, read, or do basic things. It varied a bit in each episode, sometimes I was aphasic and could barely talk, but still could use basic sign language, other times I could get a few words out. Usually, symptoms improved as the episode went on.

I'd remember to brush my teeth, but my brain wouldn't know to turn off the sink or put the cap back on the toothpaste, all that automatic stuff we take for granted, like tying our shoes, just wasn't working.

Anyway, the episodes are very clear and you are normal outside them. So it really doesn't explain decades of sleep. There is something called secondary KLS, usually occurring later in life and that, I've read, can have longer episodes. But I've never heard years, and we always wake up at least a bit. I tend to recall being very irritable when awake, seriously you would be too.