Answer:
Absence of air in uncrushed sleeping bag
Explanation:
A fluffy bag consists of air. Air is an insulator of heat and hence when a person sleeps in it, the heat of the body does not escape the fluffy bag. Due to this reason, once a fluffy bag gets heated, it remains heated for a longer period of time and the person sleeping in it does not feel the cold.
While in crushed bag, air is absent thus a person sleeping in a crushed bag loses his body heat soon and starts feeling cold.