What's the meaning of sleeping in a sack?

The moral of sleeping bag is that "sleeping bag" is just a sleeping bag. In the field trip, a good sleeping bag can bring clean, warm and comfortable sleep to tourists. Sleeping bags are mainly filled with down and artificial velvet. In terms of texture, it mainly includes canvas, nylon and waterproof cloth. The style is relatively simple, generally divided into two parts, which can be folded into a smaller volume like clothes and can be put into a backpack.