Modern technology has produced some excellent man-made fibers, which have resulted in the development of some compact and lightweight sleeping bags but there is still no comparison to a down sleepingbag when you are looking for super-light performance.  The down sleeping bag will give you the lightest sleeping bag with the most warmth. 

Because the down filled sleeping bag can be warm and lightweight,  it makes a good choice for the backpacker who needs to have a lightweight pack and a comfortable night sleep.   Down bags are highly compactible making them easier to carry with a pack full of gear but once released from their compression bag fluff up for a night of comfort and  warmth.  When weight and warmth are of great importance a down sleeping bag is the way to go.