You know, one of the most transformative things I've read about mental illnesses, specifically depression at the time, is how people who suffer from long term depression also exhibit symptoms of PTSD. It makes you realize not just how hard it is to deal with, but the sort of long term effects it has on you.
As for sleeping half the day, I'm not sure that's just the coward's day of coping, and I have to put my two cents that coping is better than, well, dying, but that these things put a lot of physical stress on us. I remember when my heartbeats per minute were 120 for a period of 3 days. You know what that does to you? It tires you out. Not to mention serotonin and other related things.
But yes, realizing a day is shot, and forgiving yourself for lack of productivity? It's not easy, and I often only get there after half a day, or 3 days of feeling terrible with myself, but that's the only thing I've found to work. Aside from "Just work anyway. Just don't be depressed!" sort of nonsense.
Thanks man. That last line sound so very familiar...
The depressed person has many crosses to bear, but that one's probably the (second) worst.