Another option between a full-on tent and a tarp is one of these 'bothy bags', which is essentially a large orange plastic bag. http://www.mec.ca/AST/ShopMEC/Tents/...on-shelter.jsp
They weigh about the same as a tarp, but offer full enclosure from the wind/rain without requiring guy outs. Add a couple of guy lines and or treking poles and you basically have a small floorless tent. An emergency candle/lantern should do a good job of supplementing your insulation.
here's a video to give you an idea of the set up: