I don’t think that there are many people who can code in a concentrated manner for more than, say, 6 hours per day. There are days when I manage 8 hours of actual coding, but those are not my favorite ones - I only do those under immense deadline pressure, and with all the interruptions it means sitting at a screen for 10-12 hours. Such days leave me exhausted for 2-3 days after. Not sure why, but the brain is a muscle after all, and it can get sore.

I got sidetracked.. what I mean is that you need HN, Facebook etc to prevent you from using too much of that precious brain power. Maybe this is different for other areas of work, but IMO it should make not much of a difference whether you are forced to stay at work for 35 or 45 hours - you just will procrastinate more.

Update: Added “more than”.