In general there are two types of sleeping bags; goose down bags and synthetic sleeping bags. These differ by the material that is used for the fill. The fill is the most vital part of the sleeping bag. It's what prevents you from losing heat. The fill should do a good job of trapping air, providing you protection from the cold outside air.

Down Mummy Bags

Down is the most efficient fill for sleeping bags, because it's very light and traps air better than synthetic bags. This warmth for weight efficiency makes it ideal for mountaineering.

Down Mummy Bags compress better than any current synthetics and so are less bulky when packed. When opened up, a down-filled sleeping bags fluffs back to its open state very quickly and more completely. It has almost fluid like properties as it readily fills up the void compartments of the sleeping bags.

Good quality down originates from China. A debate has been made in the past whether white goose down are better than gray goose down but there's really not much difference.

You will likely see a down sleeping bag being marked as 600-fill, 700-fill or 800-fill. This is called the fill power. Fill power is the amount of space that one oz of down will occupy.

The bigger the number of fill power means that for the same amount of space, less amount of down is used which makes the sleeping bag lighter. This is accomplished by using fluffier and longer-tendrilled down.

The American Society for Testing and Materials (ASTM) ordained standards for down content and fill power. By law, down must be at least 80 percent down i.e. cluster and fiber rather than quills, beaks, etc.

A good down sleeping bag will bounce back when you poke it with your finger. When properly taken care of, a down filled bag can last for a long time. One avid hiker I personally know used his for more than 20 a long time.

But down as a fill is not perfect. If you get it wet, it will stop keeping you warm and you'll have better luck drying mud on a rainy day than drying a down sleeping bag on your trip.

Down is a rather costly material and requires much more care in maintenance. In some cases, infrequent as it is, may cause an allergic reaction.

Synthetic Filled Mummy Bags

Synthetic fills have been around for a long time. They come in different name like Polarguard, Thermolite Micro, Hollofil I & II and more. The names frequenly change and the producers will likely come up with fancier names.

Synthetic fills are categorized into three forms. The fibers can be lubricated with silicon or other liquids to make them feel more silky, more like down.

Yet down is still the best fill for sleeping bags, synthetics are not far behind. One major advantage of using synthetics is that, unlike down, they still retain their insulating properties even when wet and drying them is very easy.

And because synthetics are a lot cheaper than down and there's still room for improvement.

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