Down sleeping bag

 If the temperature is warm, these may be the approach for you; but after the first night, they begin to fill with moisture and following nights will become progressively less effective at keeping warmth in sub zero operating climates as they turn from a sleep system to a freezing system. When you leave the system in the morning, the moisture freezes and awaits you the following night. 

The effect of moisture retention is particularly dangerous for you feet as they are so small and can only produce so much heat of thier own.

How you "dry them (the down bag) out" in freezing tempatures (below -5c) is a problem I am still working out. In freezing tempatures, the sun and air does little to remove the ice..

After four days in my case, action  must be taken to adress moisture. Produce a steady frame near and well over your fire; this seems to be the only effective way to remove moisture, dangerous to the bags at it sounds. On the fifth day you will reaize the value of this effort as your bad is just as cozy as the first.

At best, is split the layers and open the bag as well as you can in morning during breakfast and try to air them out as much as possible. Turn them inside out half way. Be warned, doing this does little in freezing tempatures.