Originally posted by rotticus:

Just build the fire on the other side of the wall your bed is against. Works for me every time.

Exactly. While you can build inside using some of the methods listed above there's no requirement for the fire to be inside the structure. It simply has to be within a certain range of the bed.

It does make some sense because it gets cold at night which causes a debuff and being near a fire cancels it. Just think of it being a bit too cold to sleep safely, and the fire ensures you don't get hypothermia.