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 due to the moisture - you will produce at night - slowly saturating the fibers. 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 figuring out. You can't put them next to fire as an ember would dammage the surface and the sun does not seem to remove the ice crystals.

An effective and prudent habbit however is while in above -5c operating temperatures (and below as well, however I haven't given this one enough practice) , 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.