Why do students usually assume faculty members are available all day long? Same students, it seems, send emails at midnight and expect a quick answer. Um, yes -- many of us (well, speaking for myself at least) work whenever we have time.
Still, I have heard negative comments in the past about how great it is to have "flexible" hours. Hey, they are not always flexible and we often work long, long hours when our students don't even realize we do. I love my job but sometimes I am actually envious of people who work 8:00 to 5:00, with a scheduled lunch break, and go home w/out worrying about work left behind.