Sleeping together or alone — Which is better? Results depend on "social rank and genetics"

A research group led by Professor Yu Hayashi and Project Researcher Naoko Hayashi of the Graduate School of Science, the University of Tokyo used social mice to show that sleep quality changes depending on whether mice sleep together or alone, and that REM sleep in particular varies greatly with social rank and genetic background. The findings are expected to advance understanding of how stress and loneliness affect sleep. The results were published in Scientific Reports on January 19.

In recent years, "sleep divorce," where partners sleep in separate rooms, has been on the rise. It has been unclear whether the "sense of security arisen from sleeping with someone else" or "comfort of sleeping alone" leads to better sleep quality.

Previous studies had noted that fighting and physical contact among mice in a group can harm sleep quality. It had been difficult to separately evaluate the "impact of social connection" itself, independent of those physical disturbances.

To address this, the researchers introduced a "neighbor-housing" condition that prevents physical contact while preserving social connection. Under this setup, mice are housed in the same cage but separated by a transparent acrylic partition, allowing social interaction through sight, smell, and sound while blocking direct aggression and physical contact.

The experiment involved switching mice from neighbor housing to single housing (isolation) and comparing dominant mice, those with higher social rank within the group, to subordinate mice with lower rank.

Under neighbor housing, where physical contact was largely absent, no significant difference in sleep duration or REM sleep amount was observed between dominant and subordinate mice.

Under single housing, however, opposite outcomes emerged. Sleep quality improved in dominant mice, while the sleep quality of subordinate mice-as measured by REM sleep brain activity-declined. For dominant mice, single housing was thought to free them from the minor disturbances and vigilance that come with sleeping together, while for subordinate mice, the loss of the security of having others nearby was believed to cause social isolation stress that worsened sleep quality.

Further investigation using two mouse lines with different genetic backgrounds showed that in B6 mice, which are commonly used in neuroscience research, subordinate mice showed an increase in REM sleep amount under single housing. In contrast, F1 hybrid mice, which form more rigid social hierarchies than B6 mice, showed no change in REM sleep amount.

The findings demonstrate three factors: The presence or absence of social connection, one's position within the group, and genetic makeup work together to regulate REM sleep, which is also believed to play a role in brain maintenance. The results are expected to contribute to understanding the mechanisms behind sleep disorders.

Yu Hayashi commented: "This study demonstrated in mice that sleep quality is influenced by the social environment, one's social position, and genetic constitution. Interestingly, when physical contact was avoided while social connection was maintained, no sleep differences were found regardless of social rank. At the same time, subordinate mice showed worsened sleep quality due to isolation. 'The right kind of connection for each individual' may be a key to achieving good sleep and brain recovery. We hope to build on these findings toward optimizing sleep environments for people in modern society and protecting mental health."

Journal Information
Publication: Scientific Reports
Title: Social rank and social environment combinedly affect REM sleep in mice
DOI: 10.1038/s41598-025-32402-2