Why do I get so tired just from sitting on my ass drawing for an hour or two? I'm in a decent spot skill wise and motivation wise but now I'm having issues with getting tuckered out too easily, I end up only doing warm ups or dumb doodles and only making some progress on proper pieces. Maybe it's cause I slouch in my chair with my bamboo or ipad on my lap, I've been trying to get out of that habit and actually sit straight.