phurley has the right idea. A good quality winter down bag should have a reinforced area at the feet for your boots and your water bottle. I have camped in some fairly cold weather in Colorado and Wyoming. On one Colorado trip we spent 21 days in a tent. Many mornings the temp was 20 degrees below zero. Do not know how cold it got durring the night as I did not open my eyes to look. Sleeping naked in a bag will not work for me, as every time I move around a little of thet cold air gets in the bag, even with the hood REAL tight. I wear polypro underware, sometimes even putting on expedition pants and a fleece jacket and a down vest. I always wear a balaclava hood that protects my head and neck. I will wear fingerless wool gloves. I put on fresh dry socks as I get into the bag. It also helps to put baby powder on your feet. My feet get cold so I have even wrapped my feet up in a fleece or down jacket in the bottom of the bag. If you use your bag quite a bit you can also get a silk liner, increases warmth a little bit and keeps the bag from getting as dirty.
On sunny days I will turn the bag inside out so as the tent heats up the bag will dry out. This is especially important on long trips and with Gore-Tex bags.
One thing I do know for sure. DO NOT skimp on your sleeping bag. You do not want to be miserable at night when you should be recharging your batteries, and if the weather takes a sudden change for the worse the bag may become your LIFE SUPPORT SYSTEM.
I have 3 different sleeping bags that I use.
One is a very lightweight Marmont Mountain down bag for back packing.It is rated as a 30 to 35 degree bag. You can shoft the down from bottom to top for summer use. It has a hood.
Second bag Is also a Marmont Mountain down bag with Gore-Tex shell rated to minus 30 degrees. I have had these two bags for many many years, both were bought in the mid 1980's.
The third bag is the newest. It is a Mountain Hardware synthetic bag. It has all the features of an expedition down bag. It is rated at zero if I remember correctly. I bought it for an Alaskan trip at the INSISTANCE of an outfitter. He said under NO conditions was I to bring a down bag to Alaska. He stated [and correctly] that if a down bag gets wet it is useless and will NEVER dry out in Alaskan conditions.
I am pretty careful with my down bags. I carry them in water proof storage bags [Outdoor Research Advanced Hydro Seal Stuffsacks]. but took his advice. Synthetic is warm when wet and will dry in the wind.

I have a fleece sleeping bag liner, pull the bag up, the liner up, and I'm out like a light

Used to but my dog inside, but it not quite the same.

Johan

After reading all the comments here, let me tell you how I handle the cold conditions, after years of hunting, backpacking and climbing. Now this is not necessarily gospel, just my way that works nicely for me. ----- First good equipment, my bag is goosedown with gore-tex shell, rated at zero. I am 5'10" and my bag is 6'6", thus allowing room in the bottom for my boots (in plastic bag) to be placed in the bottom of the bag with enough room for my goose down jacket to be placed between the boots and my feet. When I get up in the morning my boots and jacket are toasty and easy to put on. The bag has a draw string face tunnel that I pull tight but I do not breath inside the bag. I have one of those very light blankets the airlines furnish on planes that I put over my face if it is really cold. If the temp gets below my rating on my bag I place another light polar fleece blanket over the bag, if anymore cover is required I will use a light tarp over that. With this outfit I can sleep comfortably to 30 below, colder that that I try to avoid or be inside a shelter of some sort. ----- I also pack this bag in my backpack with two days supply of emergency food when Elk hunting so that if I get down, I could survive until found, hopefully. For what it is worth, this method works for me. Good shooting.