Let's start with the basics: a float tank is essentially the perfect bath tub. Air is allowed to freely flow in and out, and the door never locks or latches. It holds about 11" of water, which is saturated with 1100 pounds of Epsom salt. This creates a solution more buoyant than the Dead Sea, and you float on your back about half in and half out of the water.
The water is kept at the average skin temperature (93.5 degrees F), which allows you to lose track of your body. The tank is sound proof and, when you turn off the light, completely dark. No gravity, no touch, and no sight. Let's explore what all this does.
The buoyancy puts your body into what is essentially a zero-gravity environment: your muscles and bones get to relax, your joints and spine decompress, and the strain on your circulatory system is greatly reduced. The energy that your body usually spends fighting old man gravity is suddenly available for healing and rejuvenation. It's basically like your entire body breathing a sigh of relief.
Once you're in this state of relaxation, your brain waves lower their frequency from our waking alpha and beta states down into a theta state. This is where you go when you daydream, or when you're just drifting off to sleep. Theta waves are typically rooted in the right-hemisphere of your brain and are known for encouraging creativity and inspiration.
With no external stimuli coming in, your system stops worrying about all the background tasks that usually keep it occupied - mostly related to not dying. Your flight-or-flight response gets a chance to kick back and stop bossing the brain around, lowering your production of adrenaline and cortisol. Instead studies show signs that your dopamine and endorphin levels rise, giving you a natural mood high which often lasts for days.
Sensory deprivation effects us in another notable way; as our brains become stimuli starved, their reaction is to start creating their own stimuli. Swirling nebulas, gorillas on surfboards, and frolicking Labradors are only a sampling of the visualizations that people have reported from their time in the tank. Writers are moved to put pen to paper, musicians to put fingers to strings, and painters put brush to paper.