It sucks that you have to stay until 5. Around 3:30, I ran out of work. Not that there wasn't anything to do, but nothing sort of concrete and manageable within the 1.5 hours I had left presented itself. And I was tired. If I were a lawyer or consultant or something, I might have put the 1.5 hours in and billed it to whichever client's project (or case) I was working on. Or if I were a professor, I might have gone and worked out and put in another couple of hours after dinner. But a salaried job doesn't work that way. There's very little external motivation to work in terms of direct compensation for the hours you work. And a lot of internal motivation comes from a) enjoying what you do, b) being told you have to do something or c) the culture of the clock. If you don't enjoy your job, then b) and c) are the only motivations and those don't seem like good motivations to me.
On a day when 3:30 rolls around and I'm unmotivated, but don't have the guts to just say, "I'm done for today" I do things like surf the internet, read blogs, etc. Or, as I did today, I had a conversation with a colleague about something related to work, but which wasn't very productive. I'm think it would be better for everyone if I just left. Then I'd come back the next day refreshed and more productive. And believe me, the actual 1.5 hours of work will be made up sometime. I routinely work over my scheduled hours.