In a paper published in 2018 (Feinstein, Justin S., et al. "The elicitation of relaxation and interoceptive awareness using floatation therapy in individuals with high anxiety sensitivity." Biological psychiatry: cognitive neuroscience and neuroimaging 3.6 (2018): 555-562.), 31 participants with high anxiety sensitivity were randomly assigned to undergo a 90-minute session of Floatation Reduced Environmental Stimulation Therapy (REST). The results find that relative to the comparison condition, Floatation-REST generated a significant anxiolytic effect, characterized by reductions in state anxiety and muscle tension and increases in feelings of relaxation and serenity. 

To find results for other types of relaxation therapy, you can start from the following metareview of 16 controlled and uncontrolled study as published by the University of York. (Keren Reiner, MA, MSc and others, Do Mindfulness-Based Interventions Reduce Pain Intensity? A Critical Review of the Literature, Pain Medicine, Volume 14, Issue 2, February 2013, Pages 230–242,)
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