Being a type 2 diabetic I'm not sure if this particular writing related 'ailment' is physical or mental, but I'm looking at ways to banish my sudden sags of enthusiasm that hit me like a freight train after about three hours of writing. Admittedly it could be one of the downsides to polishing as you go, which is a habit I can't get out of of. I've accepted this means my daily word count will be low, and I'm fine with that, but I wonder if has anything to do with the sags. They hit me around the three-hour mark and can strike in the middle of a sentence, let alone a scene. When they do hit I get an urge to stop writing which is impossible to fight through. My body slumps, I feel completely fatigued and tired (as though I could curl up and sleep right there and then) and all my desire and enthusiasm seeps away. I save and shut down the document and feel a great release in doing so. I pick up a few hours later, but by then can't motivate myself to get back behind the keyboard.