You can use a regular sleeping bag when spending the night snoozing on a hammock. We doubt it's going to be all that comfortable, though -- sleeping bags are built for lying flat on a floor, not swinging on a hanging sack. This Hammock-Compatible Sleeping Bag allows you to both stay warm and lie down at the most comfortable position you can find.
Made by Grand Trunk, the bag is specially-designed to wrap around the entire hammock. That way, you're lying down right on the hammock without an extra layer of foam sandwiched between, all while your body stays fully insulated from the cold. Basically, it turns you into a hanging cocoon -- just like in one of those alien movies.
The Hammock-Compatible Sleeping Bag is made from nylon, with Thinsulate fill to keep you warm and toasty (rated to 32 degrees Fahrenheit) while you sleep a couple of feet off the floor. Dimensions are 88 x 34 inches, so it should fit most everyone who isn't a 7-foot-plus NBA center. Features include two full-length two-way zippers, draft collar, interior zip pocket and an adjustable mummy-style opening on the head. It comes with a stuff sack where you can cram the entire thing for easy portability.
If you've ever tried using a sleeping bag on a hammock and found the thing compressing under your back just a tad uncomfortable, the Grand Truck Hammock-Compatible Sleeping Bag should save you from that hassle. You can pre-order directly from the Grand Truck website, where it's priced at $179.99. Ships in May.