If you're only going overnight, assuming you bathe the day before you leave, you won't get the sleeping bag that dirty even if you have to strip (I suspect the cap will be too much). It's those multiple-day trips in which we get really greasy and ripe!

You can always zip open the sleeping bag and use it as a quilt, or even just sleep on top of it if it's that warm.
_________________________
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey