In reply to Dave the Rave:
> This will allow heat to escape to the filling and trap warm air.
Respectfully, when it comes to insulation, you want the the heat to not escape. To start with, the bag will be cooler with clothes on than when next to bare skin but that is because the clothes are insulating the bag from you. This may be affect you feel.
In the steady state (when you have been in the bag for a while) the heat power loss will be the same as the heat power you generate, clothes or no clothes. But assuming the clothes are acting as effective insulation (they are not wet) you will have a larger thermal gradient between you and the outside of the bag, meaning you are warmer.
> This clothing may also cause the person to sweat in the night which will also reduce warmth.
Surely this is contradicting the point above, why would they sweat more if the bag is not working as effectively and so keeping you cooler?