In reply to Oo:
Depends on what sort of routes you're doing, where you are going to bivvy, altitude, how many nights etc. My preference is for a very lightweight down bag ( ie; Phd Minimus or similar ) and a silk liner. This is with proviso of being well fed (fatty foods), hot drinks, good forecast, I'll have warm clothing including hat and gloves, metal water bottle as hot water bottle and a few sit ups when I first get into the bag, sheltered spot, good insulating mat, bivvy bag. Also, I'm usually getting up between 1.00 - 3.00 am so it is a fairly short night.