The sleep-promoting effect of a hot bath on daytime sleep after night-shift work was assessed by polysomnography. Having an unlimited daytime sleep opportunity, healthy middle-aged male subjects (n = 9, mean age 38.8 years) undertook either a hot bath or no bath (control) treatment. The polysomnography revealed that a hot bath significantly (P < 0.05) increased rapid eye movement (REM) sleep parameters. Also, the number of REM cycles increased from 1.7 to 3.7(P < 0.05) while REM density increased from 7.31 to 15.13 (P < 0.05). In addition to the sleep-promoting effects, a hot bath significantly improved the subjective emotional evaluation score assessed by a profile of mood states (POMS) questionnaire, especially decreasing confusion, anger and hostility (P < 0.05). The data suggests that a hot bath may be an effective and practical method of promoting sleep for night-shift workers and trans-meridian travelers attempting to sleep during the day.