Researchers tested what factors improve or worsen the quality of sleep in mice. A team including researchers from the University of Tokyo placed mice in two environments, one where they could see and sense other mice without physical contact, and one in complete isolation. They found that mice higher in their social hierarchy likely benefited from isolation, while those lower did not. However, the specific impact on the amount of REM sleep varied depending on the genetic background of the mice. The team hopes to investigate the relationship between social connections and sleep in humans.

We all like a good night's sleep. While some factors that improve or reduce the quality of sleep are relatively well understood, many are not. Given the ubiquity of sleep in society and the impact it’s known to have on matters such as stress, concentration and cognitive abilities, sleep is an important topic of study. Partly inspired by an increasingly common phenomenon called sleep divorce, where a couple elect to sleep separately if they find it improves their sleep quality, a team led by Professor Yu Hayashi from the the Hayashi Sleep Lab wanted to explore the question, “Does sleeping together and the sense of security improve sleep, does the comfort and quiet of sleeping alone improve it, or are there other factors?” This question would be very hard to study at scale in humans, so the team explored this idea in mice.

“In our latest study, we investigated how ‘rank within a group’ and being alone affects sleep in mice,” said Project Researcher Naoko Hayashi. “Like humans, mice form societies where they establish a hierarchy to determine dominant or subordinate roles. We placed mice from two genetic groups in two environments, one where they could see and smell their neighbors but were unable to fight, and another where they were completely alone. We measured their brain waves to observe differences in sleep. The results revealed that even when left alone, the changes in sleep differed significantly depending on whether the mouse originally held a high or low rank depending on its genetic type.”

The team studied two strains of mice with different genetic backgrounds, referred to as B6 and F1 respectively, to understand how genes can influence sleep response. While both strains established clear social hierarchies, the rank structure in F1 was more distinct compared to that of B6. Typically, regardless of lineage, mice of these types will sleep communally. And when placed in neighbor-housing, where the mice could sense each other but were free from being bothered by others while they slept, there was no difference in sleep patterns between dominant and subordinate mice in either strain. But when placed in full single isolation, a genetic difference emerged. F1 mice showed little change in their amount of sleep, but B6 mice experienced significant shifts in their sleep patterns depending on their rank.

“When mice were placed in complete isolation, their sleep quality changed in opposite ways depending on their original rank. Dominant B6 mice’s sleep quality improved when alone. This might be because they were freed from the stress of guarding against others or competing for territory,” said Hayashi. “Subordinate B6 mice, conversely, had worse sleep while alone. Though they had an increase in rapid eye movement (REM) sleep, it was of worse quality. This might stem from losing the security of having someone nearby and experiencing heightened stress from loneliness. In other words, this discovery shows that in mice, whether being alone is healing or stressful depends on their environment, individual social standing and genetic grouping.”

The question you might be asking is “but what about humans?” After all, the study was inspired in part by a behavior observed in people.

“Our study is just the starting point. Humans and mice are different, and human loneliness is much more complex and can last for many years,” said Hayashi. “While we haven't proven the long-term effects on humans yet, we found an important hint, that being away from others can disrupt the brain’s ability to stabilize its internal rhythms during sleep. We believe this discovery will be a big clue in helping us understand and protect human mental health in the future.”