[Purpose] Herein, we aimed to investigate the effects of bathing in a sodium chloride spring and an artificially carbonated spring on core body temperature and electroencephalograms, to assess whether the springs facilitate sleep. [Participants and Methods] This randomized, controlled, crossover study evaluated the effects of a sodium chloride spring, an artificially carbonated spring, a plain hot bath, and no bath on sleep. The subjective evaluations and recording of temperature were performed before/after bathing at 40 °C for 15 min at 22:00 h, before nocturnal sleep (0:00–7:00 h), and after the participants (n=8) woke up in the morning. [Results] Bathing significantly increased the core body temperature, with significant subsequent declines observed until bedtime. Participants in the sodium chloride spring group had the highest average core body temperature, while participants in the no-bath group had the lowest average core body temperature before bedtime (23:00–0:00 h). During bedtime (1:00–2:00 h), the participants in the no bath group had the highest average core body temperature, while participants in the artificially carbonated spring group had the lowest average core body temperature. The amount of delta power/min in the first sleep cycle significantly increased in the bathing groups, with the highest value during bedtime being recorded in the artificially carbonated spring group, followed by the sodium chloride spring, plain hot bath, and no-bath groups. These sleep changes were associated with significant declines in the elevated core body temperature. Increased heat dissipation and decreased core body temperature were observed in the artificially carbonated spring and sodium chloride spring groups, which increased the delta power during the first sleep cycle compared with that observed in the plain hot bath group, followed by the no-bath group. [Conclusion] An artificially carbonated spring would be the most appropriate given each circumstance because it did not cause fatigue, as observed with the sodium chloride spring.
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