Is a 6 inch mattress thick enough?

For the right sleeper, yes. A 6 inch mattress suits children, lighter teens, bunk beds, trundles, daybeds, RVs, and occasional guest use. It is generally too thin for average-to-heavier adults who sleep on it nightly, since there is not enough material to cushion the shoulders and hips and you may feel the foundation through it. Match the height to the sleeper and the use, not just the number.