Bed-Sharing in Couples Is Associated With Increased and Stabilized REM Sleep and Sleep-Stage Synchronization

Background/Objectives: Sharing a bed with a partner is common and affects sleep quality with implications for mental health, but findings—especially with polysomnography—are scarce and mixed. This study tested how a partner's presence influences individual sleep architecture and dyadic sleep-stage synchronization. Methods: Twelve young healthy heterosexual couples underwent sleep-lab polysomnography during two arrangements: individual sleep and co-sleep. Individual sleep parameters and dyadic sleep-stage coupling (via cross-recurrence quantification analysis) were measured; subjective sleep quality, relationship characteristics, and chronotype were assessed. Co-sleep vs. individual sleep and interactions with gender, chronotype, and relationship factors were analyzed. Results: Co-sleep was associated with about 10% more REM sleep (relative to total sleep time), reduced REM fragmentation (p = 0.008), longer uninterrupted REM fragments (p = 0.0006), and more limb movements (p = 0.007); other stages did not differ. Social support interacted with sleeping arrangement such that suboptimal support showed the largest REM change. Sleep architectures were more synchronized during co-sleep (p = 0.005), including when wake was excluded (p = 0.022), and coupling extended across ±5 minutes. Relationship depth showed an additional positive main effect on synchronization. Gender, chronotype, and other relationship characteristics did not significantly affect REM or synchronization. Conclusion: Bed-sharing alters sleep architecture and increases sleep-stage synchronization, modulated by relationship characteristics. These changes may reflect a feedback loop between REM sleep and sociality and represent a pathway through which social relationships support mental health.