| Due to the accelerated pace of modern life,people’s sleep patterns on weekdays and weekends are often not consistent.Sleep on workdays starts later and ends earlier.To compensate,people choose to catch up on sleep and sleep more on weekends.This change in sleep-wake schedule during the week can lead to chronic circadian rhythm disruptions accompanied by sleep loss.Social jetlag,the difference between an individual’s endogenous biological clock and actual sleep time,is the most common form of circadian rhythm disruption and is used to represent sleep patterns that vary during the week.Social jetlag is closely related to many aspects of human physiological,psychological and behavioral problems.Some studies have shown that higher social jetlag leads to more impulsivity,risk-taking behavior,substance use,etc.,which may be related to impaired reward function due to high social jetlag.In addition,social jetlag is often accompanied by chronic sleep loss.Sleep loss is also associated with impaired reward function.For example,studies have shown that sleep loss decreases positive mood and leads to more depressed mood,substance use,and risk-taking behavior.The underlying neural mechanism behind the poor health problems caused by social jetlag and sleep loss may be impaired reward function.Reward functions affected by social jetlag and sleep loss mainly include reward processing and reward decision making.Reward processing is a complex process that mainly includes reward expectations(want component)and reward outcomes(like component).Reward decision making,also known as risky decision making,involves weighing potential rewards and possible risks under conditions of uncertainty to make the best decision and obtain the maximum reward.Behaviorally,both social jetlag and sleep loss lead to altered mood and more risk-taking behavior;physiologically,numerous resting-state and task-state functional magnetic resonance studies(fMRI)have shown that high social jetlag affects reward processing capacity,for example,greater weekend-workday sleep midpoint time variation is associated with reduced striatal activation during reward processing.Less sleep time is also associated with altered activation in the striatum and anterior cingulate cortex during reward processing and leads to reduced insula activation during reward decision-making.Although social jetlag was indeed found to affect reward function in previous studies,these studies often confounded the different effects of social jetlag and sleep loss on reward dysfunction.Therefore,the present study focused on the different effects of social jetlag and sleep loss on reward function,for which subjects were recruited using social jetlag = 1h and sleep duration = 7h as the basis for division into a control group(sleep duration ≧ 7h and social jetlag < 1h),a sleep loss group(sleep duration< 7h and social jetlag < 1h),a social jetlag group(sleep duration ≧ 7h and social jetlag ≧ 1h)and a sleep-deprived co-social jetlag group(sleep duration <7h and social jetlag ≧1h).The following three experimental studies were conducted:Study 1 used questionnaire data from the Behavioral Brain Research Project of Chinese Personality to compare the differences in scores on the Reward and Punishment Sensitivity Questionnaire,the Positive and Negative Affect Scale,and the Depression Self-Rating Scale among the four groups of subjects by one-way ANOVA.Post hoc analyses showed that the sleep loss group and social jetlag group had significantly higher depression scores and punishment sensitivity scores compared to the control group;the sleep-deprived co-social jetlag group had significantly higher depression scores,reward sensitivity and punishment sensitivity scores compared to the control group,and significantly lower positive emotion scores;the four groups did not show any significant differences in negative emotion scores.This result supports our conjecture that sleep loss alone and social jetlag alone do affect individuals’ emotional states,and that the combined effect of sleep loss and social jetlag is more detrimental to emotional states.Study 2 recruited subjects through online advertisements and used a slow eventrelated nuclear magnetic guessing game paradigm designed to examine the neural responses of four groups of subjects to monetary reward anticipation and outcome feedback.Each trial included a reward anticipation phase and an outcome feedback phase,in which participants learned whether the current trial was a reward trial or a loss trial and inferred the likely outcome accordingly;in the outcome phase,they received feedback on whether they won,lost,or remained unchanged.Based on previous studies,data analysis focused on rewarding trials,with the ventral striatum and medial prefrontal lobe selected as regions of interest for activation analysis in a one-way ANOVA,and whole-brain activation was also explored.The results of brain imaging showed that we were not able to find any between-group differences within the two regions of interest during the reward anticipation phase,but in the whole-brain activation analysis,the four groups of subjects differed in activation in the middle frontal gyrus and cerebellum.Post hoc analyses showed that the sleep loss group showed higher middle frontal gyrus activation and the social jetlag group showed higher cerebellar activation compared to the control group.In the reward outcome phase,we found significant group differences only in the ANOVA with the ventral striatum as the region of interest.Post hoc analyses showed that activation in the ventral striatum was significantly lower in the sleep deprived,social jetlag and sleep deprived co-social jetlag groups compared to the control group.The aforementioned brain regions were shown to be involved in the assessment of reward or the modulation of response to reward,which could serve as evidence for a neural mechanism of impaired reward processing due to sleep loss and social jetlag.Study 3 used a balloon analogue risk task to investigate whether social jetlag and sleep duration affect reward decisions and their neural mechanisms.The task was 10 minutes long,and subjects were asked to obtain as many rewards as possible by continuously inflating the balloons without making them explode.Each balloon was actively chosen by the subject to inflate or to redeem the reward,and the choice to inflate was likely to result in a larger reward,but the balloon was also likely to explode and lose the available reward.However,no between-group differences were found in the behavioral metrics of this task,which may be due to the fact that the nuclear magnetic version of the balloon simulation risk task is less sensitive to detecting individual differences in behavioral performance.In the brain imaging analysis,exploratory whole-brain activation analyses were conducted for the choice inflation,choice cash-out and balloon explosion events.No between-group differences in brain regions were found during the choice cashing events.In the choice inflation event,we found that the cerebellum,anterior cingulate cortex and precuneus showed significant between-group differences.Post hoc analyses showed that individuals with sleep loss exhibited higher anterior cingulate cortex activation compared to controls,with or without concomitant social jetlag;whereas the sleep loss group,social jetlag group and sleep loss co-social jetlag group all exhibited higher activation in the precuneus.During the explosion event,we found between-group differences in brain area activation in the cingulate cortex and subparietal lobule.Post hoc analyses showed that the sleep loss group showed higher activation in both anterior and middle cingulate cortex compared to the control group;the social jetlag group showed higher activation only in the subparietal lobule compared to the control group.Overall,sleep loss and social jetlag,and the combination of both,lead to mood changes and changes in reward and punishment sensitivity,and these changes in cognitive function may be due to impaired neural reward function.Further neuroimaging results corroborate this finding.The combination of sleep loss and social jetlag may lead to altered activation of reward/executive control brain regions including the striatum,cingulate cortex and prefrontal lobes,suggesting an underlying altered neural mechanism of reward function.The above results can help us further understand the relationship between sleep loss and social jetlag and reward function,and broaden our understanding of the effects of social jetlag and sleep loss on physical and mental health. |
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