There are a variety of hammock designs that use spreader bars including bridge hammocks. The problem is that no matter how you spread the ends of a regular hammock it will still sag in the middle, causing you to look like a banana if you sleep straight. By sleeping on the diagonal, across the curve, you can find a place that is, if not completely flat, pretty darn close. In contrast, bridge hammocks work like a suspension bridge with the support, and its curve, above you.