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Many different insulating materials are available for sleeping bags. Avid outdoorsmen usually prefer either synthetic fill or down, and they have debated the merits of these materials for years.
Synthetic fill does not readily absorb water, dries easily, and provides some warmth even when thoroughly soaked. These properties may save the owner’s life if, for example, the sleeping bag is accidentally dropped into water on a cold day. Synthetic material is also firm and resilient, so it insulates well even underneath a person’s weight. Synthetics also have the ability to loft faster than down, allowing the sleeping bag to provide the insulation faster than a down bag. On the flipside, synthetic fill cannot be compressed as much as down, causing such bags to take up more space when not in use.
Source: en.wikipedia.org