The role of sleep in cognitive and affective biases associated with depression

Abstract

Both sleep disturbances and cognitive and affective biases (attention, memory and interpretation biases) are associated with the onset and maintenance of depression. In recent decades, it has been hypothesized that sleep disturbances might be critical for the affect regulation in depression. Experimental studies further suggested that cognitive and affective processing, including attention, memory, and interpretation of emotional information in healthy populations is affected by sleep. However, it remained unclear whether and how sleep is associated with the cognitive and affective biases in individuals with depression. The current thesis reports two studies that investigated the relationship between sleep and cognitive and affective biases in depression. Study 1 was conducted to investigate the interactive effects of depression and sleep disturbances including insomnia and short sleep duration on cognitive and affective biases. Fifty-four individuals with major depressive disorder and 81 non-depressed controls completed a set of questionnaire and a one-week sleep diary with actigraph before the experiment. On the experimental day, they were administered a dot-probe task (to measure attentional bias), an incidental memory task (to measure memory bias) and an emotional face perception task (to measure interpretation bias). Study 2 involved the experimental manipulation of sleep using a napping paradigm. The same participants from Study 1 completed the aforementioned tasks again after one of the three randomly assigned conditions (i.e. No nap, 30-min nap, 90-min nap). Changes in task performance across wakefulness and different periods of sleep were compared. Results of Study 1 indicated that cognitive and affective biases in individuals with depression were moderated by habitual sleep duration and insomnia independently. Individuals with both depression and short sleep displayed more intense perception of mildly expressive sad faces than the non-depressed individuals. The recognition memory of positively-valenced pictures was poorer in individuals with both depression and insomnia compared with those with depression only. Results of Study 2 showed that within the depressed group, increased negative attentional bias was found after a period of wakefulness, whereas napping mitigated the changes. While consolidation of positively-valenced eyes pictures in the sleep condition was found in the control group, the depressed group failed to show consolidation of positive stimuli over sleep. Instead, the depressed group showed memory consolidation of neutrally-valenced stimuli after nap conditions without Rapid-Eye-Movement (REM) sleep, but not those with REM sleep. Increased intensity ratings of fearful and angry faces were observed in the depressed group who achieved REM sleep only. The findings suggest that the emotional processing in the depressed group was altered by a period of sleep. In summary, the current thesis demonstrated that cognitive and affective biases in depression were moderated by habitual sleep disturbances and instantly affected by a period of short sleep. Our findings not only provide further evidence to the role of sleep in depression, but also highlight heightened cognitive and affective biases as potential mechanisms. Future research could examine evidence-based models for conceptualization of depression and interventions focusing on cognitive and affective biases alterations as well as sleep enhancement.