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 subzero 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 your feet, as they are so small and can only produce so much heat of their own. Which, counter to thought, after the bag has warmed, take off your socks if they are wet. In spite of it freezing outside the bag.

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

After four days in my case, action must be taken to address moisture. Produce a steady frame near and well over your fire; this seems to be the only effective way to remove moisture, as dangerous to the bags as it sounds. On the fifth day you will realize the value of this effort as your bag is just as cozy as the first. Dry them out like this every day, along with following the 3x what you have on top, under your body, and you'll be able to endure some pretty cold temperatures.

In warmer conditions, split the layers and open the bag as well as you can in the morning during breakfast and try to air them out as much as possible. Turn them inside out halfway. Doing this does little in freezing temperatures.

Show some support - https://buymeacoffee.com/ropesuspenders