UHAB-1: Physiological responses to a 48-hour spaceflight analog mission underwater.

Underwater habitats (UHABs) are a promising research platform to investigate human performance in spaceflight because they combine isolation, confinement, limited physical activity, and technical life-support demands with real environmental hazards. UHABs accurately replicate several challenges of long-duration spaceflight, including real over perceived isolation, dependency on habitat and technology for survival, and a platform for evaluating novel countermeasures. Here we report the first physiological case study from a 48-hour, single-occupancy underwater habitat (UHAB-1), designed as a proof-of-concept to assess feasibility, life-support constraints, and monitoring strategies for future multi-crew analog missions. With an increasing number and more diverse groups of humans traveling to space, there is a rising demand for high-fidelity analogs and novel habitat solutions for research and training. Physiological monitoring demonstrated increased heart rate and respiratory rate, as well as reduced heart rate variability coincident with elevated ambient CO₂ concentrations (mean ∼3500 ppm, with head-level pockets exceeding 6000 ppm). This highlights the importance of robust ventilation and air mixing in protecting cardiovascular function. Ambient temperature and humidity remained stable, with minimal fluctuations in skin temperature (35.4-35.8 °C). All physiological parameters stayed within clinically normal ranges over 48 hours, but persistently elevated CO₂ highlights a critical risk for longer or more demanding missions. These findings demonstrate both the feasibility and limitations of UHABs as analogs for human research. They provide foundational insight into physiological adaptation in confined underwater environments, emphasizing the importance of integrating active ventilation, improved air mixing, and CO₂ scrubbing into future habitat designs to protect astronaut health and performance.