I go through phases... sometimes I could sit at the computer for hours and be perfectly content, and other days I'm hardly on the computer at all except to check my mail.
Writing is that way too... There are some days where, if I have nothing I need to do, I get going and can write all day. It usually takes me about an hour to write a page, because I'm so anal about it and get distracted easily. Yet there are days where I can knock off ten pages, which means at the end of the day, I've spent about ten hours writing. That's when everything is flowing, and I'm happy, and it's great, and I write either until I finish the part or can't keep my eyes open anymore, and then I go to bed, and I get up in the morning, excited about writing some more, and... nothing. I get nothing done the next day, even if all the other conditions are exactly the same. Why is that? It's so frustrating LOL.