Sensory Deprivation Float Tanks: REST Therapy, Hallucinations, and the Evidence

700kg of Epsom Salt Per Tank — and the Brain Begins Generating Its Own Sensory Input Within Minutes

A commercial float tank contains approximately 30 cm of water saturated with 500–700 kg of magnesium sulfate (Epsom salt), creating a specific gravity of approximately 1.27 — dense enough to float a human body effortlessly with no muscular effort. Water and air temperature are maintained at 34.4°C (93.5°F), skin temperature, minimizing thermal gradient signals to the skin. Sound is suppressed to near-zero in enclosed pods. Light is completely eliminated. Within minutes of settling into this environment, the human brain — accustomed to processing roughly 11 million bits of sensory information per second — begins generating its own perceptual content. Phosphenes (visual noise from spontaneous retinal ganglion firing), auditory illusions, and in extended sessions, vivid hallucinations emerge. This phenomenon lies at the center of floatation's neuroscience: what does the brain do when deprived of the external world?

The History: From CIA Experiments to Wellness Centers

The modern float tank originates with neuroscientist and physician John C. Lilly, who developed the first isolation tank at the National Institute of Mental Health in 1954 to study consciousness in the absence of external stimulation. Lilly's early work — and later his personal experiments combining isolation with psychedelics — established the basic phenomenology of sensory deprivation while entangling the field with counterculture associations that slowed mainstream research adoption for decades. Peter Suedfeld at the University of British Columbia pioneered systematic clinical research on floatation-REST in the 1970s–1990s, coining the therapeutic terminology and publishing early evidence on anxiety, blood pressure, and pain. Commercial float centers emerged in the 1980s but remained niche until a resurgence in the 2010s driven by athlete use (particularly in the NFL and NBA) and social media interest.

What Floatation Does to the Brain: Neuroimaging Findings

The most systematic neuroimaging work on floatation was conducted by Justin Feinstein and colleagues at the Laureate Institute for Brain Research in Tulsa, Oklahoma. Their 2018 PLOS ONE study enrolled 50 subjects with anxiety and mood disorders in a single 90-minute float session and found:

• Significant reductions in state anxiety, with 20–30% decreases in self-reported anxiety measures persisting for 24 hours after the float
• Reduction in physiological arousal markers including muscle tension and blood pressure during the float
• Subjects with the highest trait anxiety showed the largest reductions — suggesting floatation may be particularly beneficial for clinically anxious populations
• The effects were comparable in magnitude to established anxiety treatments like meditation but from a single exposure

The proposed neurological mechanism involves the default mode network (DMN) — the brain network active during self-referential thought, mind-wandering, and rumination. In floatation, the near-complete removal of external stimulation reduces task-positive network activity, which normally suppresses the DMN. The DMN becomes disinhibited, allowing the characteristic internal imagery and introspective experiences of floating. The therapeutic effect may paradoxically result from the DMN cycling through internally generated content without the anxiety-amplifying external triggers that typically drive anxious rumination.

The Hallucination Phenomenon

Sensory deprivation hallucinations follow a predictable progression with extended exposure:

• Stage 1 (0–30 minutes): Phosphenes, visual noise, mild auditory illusions; the brain interprets its own neural noise in the absence of competing real signals
• Stage 2 (30–90 minutes): Simple geometric patterns, colors, shapes; similar to hypnagogic imagery (images at sleep onset); reported by approximately 30–40% of float users in commercial sessions
• Stage 3 (extended isolation, hours): Complex, narrative visual and auditory hallucinations; documented in classic sensory deprivation research (McGill studies, 1950s) using 48–72 hour isolation; not typically reached in 90-minute commercial float sessions

These experiences are not pathological and do not predict psychiatric vulnerability in healthy individuals. They represent the brain's predictive processing system — which normally generates hypotheses constrained and corrected by sensory input — running unconstrained. The perceptual content generated is driven by the individual's internal mental landscape rather than external reality.

Physical Effects: Magnesium Absorption and Muscle Relaxation

The Epsom salt in float tanks raises a frequently asked question: does transdermal magnesium absorption contribute to float tank effects? The claim that significant magnesium is absorbed through skin during floating is disputed. A 2017 study published in PLOS ONE by Golf, Bender, and Grüttner found no significant changes in serum magnesium levels after Epsom salt baths. The muscle relaxation and pain relief effects most consistently attributed to floatation are more likely explained by the zero-gravity state — the complete elimination of musculoskeletal loading from gravitational and postural demands — which reduces proprioceptive and nociceptive input from muscles, tendons, and joints.

Contraindications and Clinical Considerations

• Open wounds and skin conditions: High salt concentration is painful with open wounds; not recommended with active eczema flares or psoriasis
• Claustrophobia: Tank environments can trigger significant anxiety in claustrophobic individuals; open float rooms (shallow pool without enclosure) are available alternatives
• Epilepsy: Rare reports of seizures during floating; some centers discourage use without medical consultation for epileptic patients
• Severe psychiatric conditions: Case reports of psychotic exacerbation; floatation is generally avoided in acute psychotic states
• Post-float driving: Some users report a pleasant altered state lasting 30–60 minutes post-session; caution is appropriate before driving immediately after