A significant part of my job is being available. I can leave a few minutes early if I check my email before the next day starts (ignoring social pressure; nobody's usually going to have questions at the very end of the day that can't wait for morning) or come in a half hour to occasionally an hour and a half late (nobody has immediate questions that early in the day), but mostly I need to be around during the times that everyone else is working.

If I'm having a slow period, or if I'm stuck and need time to percolate, I'll bring a book or spend more time than usual on HN.