Water-based passive body heating (PBH_WB) as a warm shower or bath before bedtime is often recommended as a simple means of improving sleep. We searched PubMed, CINAHL, Cochran, Medline, PsycInfo, and Web of Science databases and extracted pertinent information from publications meeting predefined inclusion and exclusion criteria to explore the effects of PBH_WB on sleep onset latency (SOL), wake after sleep onset, total sleep time, sleep efficiency (SE), slow wave sleep, and subjective sleep quality. The search yielded 5322 candidate articles of which 17 satisfied inclusion criteria after removing duplicates, with 13 providing comparable quantitative data for meta-analyses. PBH_WB of 40–42.5 °C was associated with both improved self-rated sleep quality and SE, and when scheduled 1–2 h before bedtime for little as 10 min significant shortening of SOL. These findings are consistent with the mechanism of PBH_WB effects being the extent of core body temperature decline achieved by increased blood perfusion to the palms and soles that augments the distal-to-proximal skin temperature gradient to enhance body heat dissipation. Nonetheless, additional investigation is required because the findings regarding PBH_WB are limited by the relative scarcity of reported research, especially its optimal timing and duration plus exact mechanisms of effects.