To me, the blues is an infection. I don't think it's necessarily a melancholy thing; the blues can be really positive and I think I think anyone and everyone can have a place for the blues. It need not always a woeful, sorrowful thing. It's more reflective; it reminds you to feel.
At the very end of a book I can manage to work for longer stretches, but mostly, making stuff up for three hours, that's enough. I can't do any more. At the end of the day I might tinker with my morning's work and maybe write some again. But I think three hours is fine.