Sometimes I'll get into 'productive' first thing, at 10amish. Other times it takes a few hours, so it's random.

Once I get into productive mode I'd rather not get out of it until I'm finished, and leaving my desk for more than a few minutes, or having to concentrate of complex tasks like talking to someone at a counter, is sure to do that.

you might stop feeling hungry, but you'll then get mysterious headaches or a sense of confusion and frustration you can't identify.