In reply to ImperialJohn:
Obviously it all depends on the weather, distance and terrain you are travelling over, the comfort level you are willing to put up with, personal metabolism, where you choose to stay the night, how much you are willing to spend etc. However, here are some things that may help you lighten up based on my experience and learning from many other people bivvying / camping in all sorts of terrain, in particular between two and three thousand five hundred metres in the European Alps.
1) Bivvy bag if solo, two person tent if not. Single skin tent only if in dry and cold environment. Two person bivvy sack for more technical routes.
2) Thermarest z lite trimmed to my body shape and cut to 3/4 length or a 3/4 length inflatable mat. If z lite is too bulky and inflatable too risky ( eg; punctures from glacial gravel ) for certain routes then a standard foam mat cut into thirds / quarters and joined by gaffa tape so it can be folded rather than rolled works well.
3) Silk liner.
4) Wear hat, socks, base layer and gloves overnight.
5) Hot drink before sleep. Fatty food before sleep.
6) Hot water put into metal water bottle kept inside sleeping bag overnight.
7) Do some sit ups when first getting into bag.
8) Place spare warm clothing over the top of sleeping bag. If windy consider 'wearing' hard shell over legs.
9) Keep everything to hand for a brew in the morning ( or overnight if required ) so you don't need to get out of sleeping bag.
10) Consider pee bottle if you are likely to need to get up in the night.
11) Body on z lite, head on rucksack to act as pillow and there is an insulating mat in the back panel
12) Try not to breathe into the bivvy bag, keep an air hole by your mouth / nose
13) Choose as sheltered a spot from the wind as possible, Consider camping lower and getting up early to catch dawn from the summits.
14) All above allows me to really reduce sleeping bag size and weight. Typically a high quality 2 season down bag such as Rab / Mountain Equipment / Phd. ( Something around 600 g. )
15) Keep walking / climbing late and get up early.
16) Eat and drink as well as you can during the day.
17) All above means I can usually get away with a sub one kilo 45-50 litre bag ( bear in mind this usually has climbing kit in too )
Hope you find something useful in this lot,
M