Plain and simple. Turn it off when you go to bed. Your DVR will last a couple extra years.
To answer your quesiton, yes you should still turn it off. If you don't only after 4 hours if no button pushes will it spin down, not off the hard drive and stop buffering. That's 4 hours of buffering that you didn't need.
This was discussed on the recent Tech Chat. They recommended shutting off the receiver at night to increase the life of the hard drive. They claimed that when a drive is in a computer, it is not really being used that much, even if the computer is left on 24/7. However, in a DVR, the drive is constantly buffering the video - so the drive is in constant use. That is why they say to shut it off at night.