I usually keep my PC running 24 hours a day as well, however I turn it off during exam time and test time during the night... apparently the more silent it is the better sleep you get (at least that what I heard once, but to me it looks like it's kind of true). So if I have a big test then I usually turn it off so that I might get better sleep and hopefully pass the test.

On other days, I keep it on and use it as an Alarm clock, so that I can wake up to the exact song I want to. This is kind of better than an alarm clock so I need to keep my PC on. I could turn it off... but I just like to keep it on .