Objectives: To preliminarily assess the effects of a single warm-water bath (WWB) on the quality of sleep, we measured sleep pattern after WWB in healthy volunteers. The primary objective of the present before-after study was to evaluate whether a single 10-minute WWB at 41°C could modulate sleep pattern in a single group of healthy subjects. In this pilot study, we also assessed the difference in general fatigue and subjects’ satisfaction responses to WWB under two conditions: WWB using tap water (WWB with tap water) and WWB using a bath additive that included inorganic salts and artificial carbon-dioxide (CO2) (WWB with ISCO2). Methods: Eleven healthy volunteers aged 20 to 48 years (29.8±8.9 years, mean ± SD) participated in this study. Inclusion criteria were as follows: age 20-50 years; free of cardiovascular disease; not taking medications or supplements. In this within-subject, two-way crossover study, all subjects underwent WWB with tap water or WWB with ISCO2 in random order for two consecutive nights. Objective sleep measures from sleep sensor mat (sleep-scan) and subjective subjects’ reports were collected. This study was approved by the Ethics Committee of Kagoshima University Hospital and written informed consent was obtained from all of the subjects. Results: None of the subjects experienced discomfort before, during or after the study period. The objective sleep measures and subjects’ reports were completed safely in all subjects. WWB with ISCO2 bathing produced significant improvement in objective and subjective sleep latency compared with WWB with tap water bathing (P<0.05). Sleep-scan-determined wake time after sleep onset (WASO), sleep efficiency, and number of awakenings (NA), and patient-reported measures of WASO, NA, sleep quality, sleep depth, and daytime functioning significantly improved following WWB with ISCO2 bathing versus WWB with tap water bathing (P<0.05). WWB with ISCO2 bathing also increased deep sleep time and sleep score (P<0.01 for both comparisons), but did not alter REM or slow-wave sleep. Conclusion: In conclusion, in our group of healthy volunteers, a single warm-water bath was shown to have the potential to modulate the quality of sleep. These findings demonstrate that WWB with ISCO2 bathing might be effective in improving some domains of sleep quality of healthy volunteers, and the subjects showed acceptance towards the intervention. Strengths and limitations of the present study as well as suggestions for further studies were considered. Further evaluations with larger and longer-term randomized double-blind placebo-controlled trials based on the present study are needed.