<p>The interplay of sleep quality, social hierarchy, and social isolation remains elusive. We evaluated such interplay using two mouse lines: C57BL/6J (B6) mice with relatively weak social hierarchy, and ICR×B6 F1 hybrid mice with relatively robust social hierarchy. Considering the potential effects of group housing on sleep - both through direct physical contact and other social interactions, which complicates interpretation—we designed a neighbor-housing condition that eliminates effects of direct physical contact while preserving social context. Under this condition, sleep architecture did not differ significantly between dominant and subordinate mice of either line. Under the single-housing condition, sleep differences emerged, some of which depended on both social rank and mouse line. In both mouse lines, single housing had opposite effects on oscillatory activities during sleep between dominant and subordinate mice. Notably, single housing significantly increased rapid eye movement sleep (REMS) amount only in subordinate B6 mice, but not in subordinate F1 hybrids or dominant mice of either lines, suggesting a genetically modulated sensitivity to social conditions. Our findings suggest complicated interactions between social environment, social hierarchy, and genetic factors in REMS regulation.</p>