My wild camping in winter tends to be in snow holes or bothies, tents tend not to last very long on the Cairngorm Plateau in the middle of winter. What I've learnt is that your water bottle makes a great hot water bottle. Before you go to bed melt some snow to fill your water bottle and put the bottle in your sleeping bag. It warms your sleeping bag up prior to you getting in and you have a supply of unfrozen water in the morning. Just don't get it mixed up with your pee bottle. Sleep with your mouth and nose out of the sleeping bag otherwise you'll breath lots of moisture into it, reducing it's efficiency - wear a hat if required.