The stress free environment allows your body turn down its “flight or flight” response, allowing the creative, relaxed part of your brain to come out and play. Without the constant pressure of analyzing the world around you, your body lowers its levels of cortisol, the main chemical component of stress.
Not having to fight gravity lets your muscles, joints, and bones take a well-deserved break. Your body suddenly has loads of extra resources (usually spent trying to keep you alive), which it gets to focus on things like healing and resting.
Without gravity pushing you down all the time, chronic and acute pain is relieved, and your muscles get to fully rest. Unlike lying on a mattress, lying in water allows blood to flow freely all throughout your body. There’s no need to readjust your position to get comfortable.
Research shows that about 40 minutes into your float your brain stops producing its normal Alpha and Beta waves and starts going deeper into a Theta and even Delta state. The real question is… Why not float?