Inside or Outside?
Yes indeed, that is an often debated question.
Some people claim that it is best to sleep with only a thin layer of clothing on your body and spread warmer clothes ON TOP of the sleeping bag.
Oh, I can't tell you how often I have heard this myth.
Don't listen to those twerps. I tell you straight, those people have never slept in a sleeping bag. Unless, perhaps, it was a sleeping bag made of pure velcro.
We are talking about winter camping and sleeping bags for backpacking and outdoor purposes and not a freaking feather bed. The outer hull of sleeping bags are made of thin synthetic materials. Which make for a very smooth and slick surface. Put something - a jacket or towel - flat on top of your sleeping bag and it will slide off as soon as you start moving and turning. Even a large, heavy blanket or a broad scarf will slide off, rather sooner than later, when you start twisting and moving during sleep. Then you will wake up because you are getting cold during the night, start fumblinge around in the dark for all those things - blanket, clothing, whatever - that slid off, try piling them back on, but they will slide off again and so on...You'll spent the rest of that night artfully stacking and balancing clothing items on top of your sleeping back, like a game of Jenga, rather than sleeping and getting some rest.
If you are cold in your sleeping bag, than the same trusted and tested layering-principel applies as for you cold weather clothing: put on another layer.
During winter camping adventures, when temps fell below -20°C during the night, we wore our ski pants and goretex jackets inside our sleeping bags. Yep, at those
temperatures our bags were reaching their limits.
No, it sure isn't comfortable to sleep in stiff goretex ski pants and goretex jacket, I grant you that. Not when you are inside a sleeping bag, that is, quite possible, already crowded with electronic gear and other items that must be protected from low temperatures. But hey..., what you do for survival you do. Stop bitching or stay home.
I remember I wore two pair of long underpants: one fleece and one wool. Long winter socks. My goretex pants. On my upper body I wore a tanktop, t-shirt, thin wool long sleeve shirt, synthetic sweater and a synthetic fill jacket. Woolen gloves on my hands and a woolen balaclava over my head. Now, this is freaking serious layering we are talking about! It got me through the night and I was ready to embark on another day long, sled-pulling adventure the next day.
Advice No. 1: If you are cold in your sleeping bag, put on more clothes. Just don't lie there, freezing your ass off. Particulare wear a hat (not a cowboy hat, of course, but a beanie) or a balaclava, as a lot of body heat is lost from the head.
Advice No. 2: Before you crawl into your bag, put on dry clothes! At the very least put on dry socks. Don't wear those you have worn all day. Even though they may feel dry, there may still be enough residual moisture from sweat on the fibres to make you feel cold. If you have moist clothes, stick them into the food end of your sleeping bag, so that they can dry off in the warmth inside your sleeping bag. Then in the morning, when you put them back on, they are dry.
Of course, the best advice still is: Get a warm sleeping bag, which comfortably covers the lowest temperatures that can realistically be expect on your adventure.