Human prosocial preferences are related to slow-wave activity in sleep.

Prosocial behavior is crucial for the smooth functioning of society. Yet, individuals differ vastly in the propensity to behave prosocially. Here we try to explain these individual differences under normal sleep conditions without any experimental modulation of sleep. Using a portable high-density EEG we measured sleep data in 54 healthy adults (28 females) during a normal night's sleep at participants' homes. To capture prosocial preferences, participants played an incentivised public goods game in which they faced real monetary consequences. Whole-brain analyses showed that higher relative slow-wave activity (SWA, an indicator of sleep depth) in a cluster of electrodes over the right temporo-parietal junction (TPJ) was associated with increased prosocial preferences. Source localization and CSD analyses further support these findings. Recent sleep deprivation studies imply that sleeping enough makes us more prosocial; the present findings suggest that it is not only sleep duration, but particularly sufficient sleep depth in the TPJ that is positively related to prosociality. Because the TPJ plays a central role in social cognitive functions, we speculate that sleep depth in the TPJ, as reflected by relative SWA, might serve as a dispositional indicator of social cognition ability, which is reflected in prosocial preferences. These findings contribute to the emerging framework explaining the link between sleep and prosocial behavior by shedding light on the underlying mechanisms. Significance Statement Sleep deprivation reportedly hampers prosocial behavior. Yet, sleep loss is not a regular occurrence. We studied participants without experimentally manipulating their sleep and conducted polysomnography along with a prosocial economic task. We found that higher relative slow-wave activity (an indicator of sleep depth) in the right TPJ - a brain region involved in social cognition - is associated with increased prosociality. This demonstrates a novel link between deep sleep neural markers and prosocial preferences. Furthermore, our study provides evidence about a possible neural mechanism that underlies the behavioral findings of previous studies on sleep deprivation and prosocial behavior. Our findings highlight the significance of sleep quality in shaping prosociality and the potential benefits of interventions targeting sleep quality to promote social capital.