Short-term hot spring balneotherapy ameliorates sleep disorders: wrist-worn wearable-assessed sleep improvement associated with neuroimmune, tryptophan metabolic and gut microbiome alterations.

Balneotherapy is a potential complementary approach for sleep problems, but its short-term sleep effects and accompanying biological changes remain unclear. We conducted a prospective single-arm, self-controlled pilot study of 30 adults with sleep disorders, who completed a 10-day residential hot spring balneotherapy program (twice daily at 09:00 and 20:00, 30 min per session, 40-42 °C) at Tianhe Hot Spring, Sichuan Province, residing on-site and receiving identical meals throughout the intervention. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI) and wrist-worn wearable tracking. Serum neurotransmitters, inflammatory cytokines and tryptophan-kynurenine metabolites were assayed, while 16 S rRNA-based gut microbiota profiles were profiled pre- and post-intervention. Post-intervention, PSQI scores decreased, while wearable metrics indicated longer total and nocturnal sleep duration, elevated deep-sleep duration and proportion, better sleep continuity, and a lower rapid eye movement (REM) percentage (P < 0.05). Blood pressure, anxiety/depression scores, and wearable-derived sleep-stress indices also improved. Biomarker profiling revealed elevated GABA, 5-HT, 5-HIAA, BDNF, and IL-10, alongside reduced IL-1β, IL-6, and TNF-α (P < 0.05). Trp metabolism shifted, characterized by decreased Trp and increased 3-HAA and PA (P < 0.05). Structurally, the gut microbiota exhibited an increased abundance of Blautia_A (P < 0.05), with LEfSe analysis identifying post-intervention enrichment of Turicibacter and pre-intervention predominance of Agathobacter. Cross-system network analysis further established 5-HT, IL-6, and IL-10 as the core candidate biomarker profile directly correlating with these multi-domain sleep improvements. Overall, short-term residential hot spring balneotherapy may improve subjective and wearable-derived sleep outcomes, accompanied by coordinated cardiovascular, stress-related, neuroimmune, tryptophan-metabolic, and gut microbiome changes, supporting a multi-system physiological basis for balneotherapy-related sleep improvement.