I would like to have setting where the display is dimmed or turned off after a certain time while music is still playing.

My scenario is this:
1. I lie in bed and start my favorite channel or audiobook. - NP3900 in normal mode with bright display
2. I set the sleep timer to half an hour so. - Playback continues with bright display
3. Sleep timer kicks in. - Display is dimmed according to my settings

You see that during 2 - while listening to the playback - the display is in normal mode. It will not turn off and continue to play after some time.

I know that I can dim the display manually, but this means that in case I want to skip a track or so, I have re-adjust the display brightness again.