I've been wondering about this being able to function thing quite a lot recently. I'm experiencing a prolonged, treatment resistant depression, yet every day I take care of my personal hygeine, go to work, etc etc. I do that because it is actually much less effort than staying in bed all day and having people nag at me or make me go to therapy or put me in hospital. Most of the time, although I'm "present" and presentable, I'm not able to concentrate, have intrusive suicidal thoughts, get no pleasure from what I'm doing, and cannot forsee a time when things will be better or recall happier times. I think that we can develop coping strategies to see us through the bad times and "functioning" is one strategy. So Axiom, it could be your reaction to humour is one of your coping strategies,especially if laughter isn't enough to lift your mood other than in that moment.