I also do know what it's like to work 50+ a week. I am currently working a full time job I work nights and go to school.
I do the same thing... postwhore SW all night. If I didn't know most of the material already and set the curve on the exams I probably would pay more attention.
I had a teacher that absolutely hated me... i'd come in 20-30mins late, sit down in the back of the class (next to the only outlet in the room), plug in and screw around the whole night. Never paid attention. He'd always call on me for answers to questions he asked, I'd tell him the 2nd derivative was toaster squared over delicious cake... he didn't find it nearly as amusing as I did.
Then the first test came. That was the first time I put my computer away during class. First one done, grabbed my stuff and left. The next week I show up late as usual, he hands me my test with a "63" on it and congratulates me for the high score. I laughed, thought he was kidding... but yeah, that was the high score. Every other test he gave was like that.
That's the way it is in most of the other classes... screw around online, sorta pay attention, enough to correct them when they're wrong... and still get the highest grade in the class on finals.
So.... If you're at the top of the class, I wouldn't worry about it. If you're struggling, pack up the computer and pay attention. One guy that was in my class played WoW all night, he had no idea what he was doing when test time came. He is now two semesters behind.