In reply to sanguine:
I've cut a Z-lite down to 3/4 length for use when bivvying in the alps and the UK. As Neuromancer says, it can significantly cut down the bulk while remaining effective. I find it works well shoulder to knee, with my boots (and a jacket) as a pillow and my sack or rope under my feet.
I tried cutting a different mat down to two short sections which I could rejoin with velcro, but the overlap really irked me during the night, so I binned the idea. I tried this mainly for packing into a more compact sack. Went back to the Z-Lite on the outside.