I recently bought a Brazilian style cotton-canvas hammock with the intent of hanging it from the ceiling indoors. I would be drilling into 2x4, and had planned to use an eye-hook screw with a locking carabiner.
However, I'm having second thoughts after going to pick up the hardware. An employee at home depot strongly suggested against hanging the hammock in this way because over time the screws would lose their grip. I'm no longer sure if it's worth the risk of smashing my spine or head off the concrete.