iPod touch adjust volume without activating screen

Hiya

5th gen Touch - how do I adjust the volume WITHOUT activating the screen? I'm CERTAIN that, a couple of days ago, I was able to adjust the level without it switching itself on but now, without making any adjustments, it activates every single time and, at 5 secs a pop, that adds up to a LOT of wasted juice!

Doesn't matter if I use the controls on the iPod, or the ones on my cans. I thought it might have something to do with the DND settings, but I've always had that switched on and - whether I'm within the DND period or not - it makes no difference.

Cheers, folks!

Sarah