The Social Nature of Sleep

If we are to make advances in our understanding of sleep and its disorders, we have to see sleep as an inherently social process. It appears to involve just a single individual, but that is an illusion. Everything about sleep is social. The sleeping individual’s body presumes it is sleeping next to another human body—whether it be a mother and her baby or a family huddled together against the cold or two adult romantic partners. Thermoregulatory processes during sleep presume that heat regulation will occur via the presence of others and biological rhythms will naturally attune across individuals once they sleep together. Even yawning is a social signal to others that it is time to attune rhythms either in preparation for sleep or an awakening.

Sleep’s normal expression requires more than one sleeping individual sleeping together to manifest. If we study sleep in the individual brain we are not seeing its true biology or power. We have known for decades that sleep architecture varies tremendously as a function of isolated dyadic sleep patterns. Sleeping alone results in a different brain physiology and sleep architecture than does sleeping with another. In general, there is astonishingly precise sleep and biologic synchrony with bed partners as well as more REM sleep when sleeping with another. Even brain physiology is different when sleeping together. Lee et al¹ demonstrated greater daily concordance in sleep behavior is associated with greater neural concordance in default-mode network connectivity between parents and children. Moreover, greater neural and behavioral concordances in sleep is associated with subjectively reported better quality sleep.

Even our dreams are intensely social, even when we sleep alone. Most dreams have at least two to three characters all of whom are involved in emotionally significant social interactions. Indeed, social interactions are so ubiquitous in dreams that some theories of dreaming posit their very function to be to simulate social interactions so that daytime social interactions are more effective in creating cooperative social interactions.

In the environment of evolutionary adaptation, we slept in groups for warmth, protection, and sexual opportunities. In one recent example of evolutionary investigations turning up the social nature of sleep, Faria et al² developed a quantitative evolutionary model sleep-wake trade-offs, showed that if individuals altruistically sacrifice sleep to protect their group mates from danger, then certain individuals will more likely develop the altruistic behavior –namely those individuals who have greater numbers of genetic relatives in the group. They will be more willing to sacrifice their sleep because they are protecting genetic relatives. It is generally believed that our genetic ancestors practiced “female-biased dispersal” wherein females left their family of origin and went to live with their “husbands” tribe/family.

Thus, in ancestral groups, females found themselves in groups without many genetic relatives while males did not. Males were the ones who stayed awake to protect sleeping tribe members. In such cases, the genetic relatedness is reduced between social partners with respect to their maternal-origin genes, thus leading ultimately to paternal-origin genes favoring less sleep and maternal-origin genes favoring more sleep. In short, our very genes encode the social forces shaping our sleep patterns.

Despite the now-ubiquitous evidence for the social nature of sleep, the vast majority of sleep studies funded by peer-reviewed science involve lone individuals “sleeping” in bizarrely unfamiliar sleep labs and reporting bizarrely uneven dreams to sleepy but bewildered scientists in white lab coats.