Saw the other thread and thought I'd add some notes.
Keep in mind that there are varying degrees of comfort desired from a cold weather sleeping experience. Some trappers or mushers just want to grab a few hours of rest and then get back on the trail, and others may attempt to stay warm for most of the dark hours, which, in Alaska winter can exceed what's expected of a good niight's sleep in a sleeping bag.
That being said, it's my belief that as long as we keep the metabolism stoked, one should be able to maintain a high level of comfort from a cold-weather sleeping experience as long as a few basic guidelines are followed.
I always attempt to over-bag myself but without totally over-burdening myself. Take enough bag!
The bag must fit and allow for the sleeper to roll INSIDE the bag. No insulation should be compressed by the elbows or shoulders during sleep. Compressed insulation allows heat to escape more rapidly. I always hear that Wiggy bags are heavy. That is because they FIT. We cut them generous because most grown men are WIDE. I WILL NOT sell any man a regular width bag because of this unless he is VERY narrow in the shoulders.
Use a pad (or two) that REFLECTS heat and does not dissipate it. I would NEVER use an inflatable pad in the winter, for instance. Your body heats the air in the pad and the ground cools it, then you heat it, then it cools it... It will be a constant battle that you cannot win. I will usually take 2 pads or at least some kind of ground cloth under my pad.
Cot camping: Always DOUBLE your pad requirement. The air beneath a cot is the temperature of the air outside. The ground is a constant 31 or so. Therefore, sleeping on a cot in -20 temperature will mean that the -20 air will constantly be below you as well as above, robbing you of precious heat.
Therefore, choose the ground over a cot in the winter unless you've got enough pad to make up for the heat loss.
Sleep with minimal clothing on. For the same reason that we cannot just put on a bunch of layers and flop down on the snow and get a good night's sleep, too much clothing worn in a bag will stifle the body's heat from filling up the expanse (loft) of the bag. That expanse will fill with either cold or warm air; your choice! The warm air needs to meet the cold air at the outside of the bag, not anywhere inside the bag. DISTANCE FROM THE COLD is what keeps us warm. Not any amount of clothing worn within a bag.
Cotton clothing is not desirable as sleep clothing as it attempts to trap the natural perspiration that we lose during sleep. That moisture will demand to be cooled, and therefore it is better to be allowed to pass through the loft and meet the cold air at the shell of the bag.
I sleep in light fleece. Silk, nylon, polyester -- all good. Just not too much of it. Naked is great, but I don't like moving my legs to the cool part of the bag. Yikes. Just a personal preference not to sleep naked. Many guys and gals prefer it and it makes sense as long as the bag drapes properly.
Waterproofing a bag will trap moisture. Not desirable. A waterproof shell will create a damp warm environment. We are seeking a dry warm environment.
Sleep within an enclosure. Tent, tarp, snow cave, etc. It will always be a great deal warmer within an enclosure, even a slight enclosure as long as the wind is not blowing through it.
Want to warm water and heat your bag? Fine. I've never done that but it makes perfect sense, however, later in the night you will be warming the water instead of it warming you. As soon as the water reaches the ambient temperature of the bag it will be detrimental because the ambient temp inside the bag will be less than 98 degrees (body temp). Ambient temperature inside your bag needs to be above 80 degrees to sleep comfortably.
Always eat before going to the bag. I usually keep cashews next to my bag and much a handful before zipping up. If you spend enough time in the bag, you WILL get chilled because you are running out of fuel, and you will need to restoke the metabolic burner! No way around that. Cashews are easy to eat and are high in calories. Slow-burning calories!
Urinate when you must. Don't waste heat on a full bladder. I wind up emptying a jar most mornings, but I don't walk around in the middle of the night and get a chill to pee.
I always sleep with insulated "booties" on my feet and I NEVER get chilled feet in a sleeping bag. Some prefer socks, but having had my pack boots or mukluks on all day and sweaty feet, the booties allow them to dry and be unconstricted.
Always wear a balaclava (preferred) or a stocking cap, beanie, etc. I like mine to cover my eyes and come up to just below my mouth.
I usually take an article of clothing inside the bag and utilize it to stop drafts or take care of cold spots. A cold spot may be caused by an elbow or shoulder compressing insulation during the night. This will happen unless you have an extremely roomy bag.
However, a bag can be TOO roomy... Keep that in mind. But better to be roomy than tight.
Hydrated muscles will promote heat retention within the body.
Don't sit around until you are cold then turn-in to sleep. Always "jump start" the warming process by doing chores around camp or even exercising to warm up the body before taking it to the bag. Cold feet take a while to heat up inside a bag, for instance. Walk around a bit before turning in.
Make sure your draft tube is on the INSIDE of the bag! You have NO idea how many customers I get in here who don't know that! Take the time when turning in to run your hand the length of the zipper to make sure the draft tube is properly covering the zipper. It will leak a ton of warmth if not properly covered.
Sleeping warm in the cold is not rocket science, but it is not an exact science either. There are some bodies who just cannot sleep comfortably anywhere other than a heated home. There are varying degrees of cardiovascular fitness and those with circulation problems just will not sleep as warm as someone who is fit.
Women and children generally sleep cooler than men as they do not have the muscle mass that men have. Muscles generate and hold heat. Therefore, a woman will generally need a warmer sleeping bag or more ground padding than a man. Fat requires that it be heated and will to some degree insulate, but it is metabolically inferior to muscle mass. Blood flows to a fat cell and comes back chilled. Blood flows to a muscle cell and comes back at body temperature or warmer if there is activity involved. I'm no Doctor, but just reach into your shorts and feel your butt and see if it is not cooler than your thigh and you'll see the difference!
This should get a good dialogue started on the subject!