Effects Of Sleep Deprivation And Short Nap On Moral Judgment

Sleep is essential for maintaining individuals’ normal physiological and psychological functions, and people spend nearly 1/3 of their time on sleep each day. As a consequence of accelerating pace and strong pressure of modern life, sleep restriction or sleep deprivation(SD) becomes a common phenomenon and a growing number of people suffer from it. SD can not only cause negative effects on individuals’ health, but also impairs their mental performance, such as reaction time, attention, vigilance, memory, decision making and executive function, etc. Moral judgment is an evaluation process, during which people judge whether some individuals’ or groups’ actions meet the moral standards or not according to certain rules and values. A large number of neuroimaging evidence has demonstrated that the default mode network(DMN) and frontoparietal control network(FPC) are engaged in moral judgment. Several previous studies have showed that SD has negative impacts on the reaction time and the quality of moral judgment, but inconsistent results exist because of the difference of experimental materials, length and way of SD. In addition, short nap(NAP) is considered as an effective measure to overcome the negative impacts of SD, which could improve the attention and alertness, accelerate reaction time and promote task performance. However, our knowledge of the role of NAP in overcoming negative impacts of SD and effects of SD and NAP on neural activity during moral judgment is still limited. Therefore, the present study attempted to explore the issues mentioned above by conducting two experimental studies.Study 1 used the traditional behavior method to investigate the different effects between normal sleep and SD on moral judgment. Twenty-one participants were asked to conduct two moral judgment tasks, one after a night of normal sleep(NS session) and one after 36 hours of SD(SD session). Compared with NS session, participants endorsed more moral violations and significantly decreased negative moral emotion scores were observed in SD session. But there was no significant difference between NS and SD sessions on reaction time. To some extent, these results indicated that 36 hours of SD impaired the capacity of moral judgment.Study 2 explored the role of NAP in overcoming negative impacts of SD on moral judgment and their neural activity. Eighteen effective participants were instructed to complete two moral judgment tasks with functional magnetic resonance imaging(fMRI) scans, one after 36 hours of SD(SD session) and one after 30 minutes of NAP(NAP session). Behavioral results showed that participants endorsed fewer moral violations in NAP session relative to SD session, which might indicate a recovery of the ability of moral judgment, but there were no significant differences between SD session and NAP session on reaction time and negative emotion scores. Consistent with prior studies, we also found strong activity within the DMN and FPC during moral judgment. To consider dynamic changes of the DMN and FPC, the mean blood oxygen response in SD and NAP sessions were divided into early(0-10s), middle(10-20s) and late stages(20-30s) respectively. NAP session was associated with decreased activity within the DMN in middle stage, and decreased activity within the FPC in late stage when compared with SD session. Moreover, a significant FPC-DMN anti-correlation was observed in NAP session, but lost in SD session. Intriguingly, the FPC-DMN anti-correlation was correlated with negative emotion in SD session, but there was no correlation in NAP session. These results indicated that NAP could greatly alleviate negative effects of SD on moral judgment and the rebooted FPC-DMN anti-correlation may be the underlying neural substrate for the restored ability of moral judgment.Combined, the present study revealed that:(1) Compared with NS session, participants endorsed more moral violations in SD session, whereas decreased moral violations were observed in NAP session;(2) For the functional network activity, NAP session was associated with decreased activity within the DMN and FPC during middle and late stages with respect to SD session respectively. And the increased activity in SD session appeared to be compensatory;(3) For the functional network connectivity, there was a significant FPC-DMN anti-correlation in NAP session, but lost in SD session;(4) In SD session, individuals with higher level of negative moral emotion demonstrated stronger FPC-DMN anti-correlations. The present study enriched the theory of effects of SD on moral judgment. To the best of our knowledge, this study was the first to reveal the recovery effect of NAP on moral judgment and investigate the neural changes after SD and NAP from the neuroimaging perspective, which would be beneficial for deeply understanding the neural mechanism of moral judgment. To some extent, these results indicate that NAP could be an effective measure to overcome the impacts of SD and they also provide some theoretical basis and reference to solve a series of problems caused by sleep loss in daily life.