Clinical ReviewPositive affect and sleep: A systematic review
Introduction
Extensive research has documented the importance of sleep for promoting restorative processes and protecting against impairments in a range of neurobehavioral functions, including emotion regulation, immune control, and memory consolidation [1], [2], ∗[3], [4]. Moreover, deficits in fundamental aspects of sleep, including sleep efficiency (i.e., initiating and maintaining sleep) and sleep quality (i.e., feeling rested and restored upon waking), can have profound health effects that contribute to increased risks for adult morbidity and all-cause mortality [5], [6], [7], [8]. Given the significant role of sleep in psychiatric and health morbidities, it is important to advance understanding of the key factors that contribute to individual differences in sleep quality.
There is growing interest in associations between positive affect (PA) and health outcomes ∗[9], [10], [11], [12]. Positive affect can be defined as a state of pleasurable engagement with the environment that elicits feelings, such as happiness, joy, excitement, enthusiasm, and contentment [13]. Encompassing both enduring moods (e.g., affective traits) as well as short-term emotions (e.g., dynamic states), PA has been found to be robustly associated with lower morbidity and reduced mortality in both healthy and clinical populations [13], [14], [15], [16]. Moreover, growing evidence suggests that PA is an important factor affecting individuals' overall sleep. Adults who report high levels of PA exhibit improved sleep patterns ∗[17], [18]. In contrast, those who experience difficulties regulating PA report greater sleep disturbances [19].
How might PA influence sleep? Pressman and Cohen [9] propose two general mechanisms—main effects and stress-buffering—by which PA can promote health. In the main-effect model, PA impacts behaviors relevant to health in general, irrespective of its effects on stress responses. For example, individuals with high trait PA may be more likely to engage in restorative health practices such getting sufficient and restful sleep [17]. By contrast, in the stress-buffering model, PA may act to reduce negative appraisals of stress and facilitate adaptive coping. Individuals with high trait PA may cope more effectively with stressors and, therefore, may not experience the adverse health consequences of stressor exposure vis-à-vis poor sleep quality [20]. Studies also suggest that PA and sleep are associated through a bidirectional relationship ∗[17], ∗[19]. For example, reciprocal inverse relations between vigor and insomnia were reported in a longitudinal study of working adults [21]. Regardless of whether PA influences sleep through direct, stress-buffering, or bidirectional effects, no systematic review has yet investigated the association between PA and sleep. Moreover, while a number of reviews have focused on links between general emotion and sleep ∗[3], ∗[22], results to date have raised important methodological questions, such as the directionality of emotion effects, the equivalence of standard subjective and objective measures of sleep, the contribution of PA to both resilience-enhancing and vulnerability-inducing sleep outcomes, and the extent to which associations between PA and sleep are independent of negative affect (NA).
Section snippets
Scope and organization of the review
To gain greater insight into the role of PA in sleep, the review is narrative rather than quantitative. Our goal in this review was to summarize research that assessed the relationship between PA and sleep. We use systematic methods and standardized procedures [23], [24] for locating and evaluating the relevance and quality of included studies. Specifically, the review includes investigations of the association between PA and sleep in healthy populations. We consider the association between
Database sources and searches
The review was conducted using PRISMA (preferred reporting items for systematic reviews and meta-analyses) guidelines [23]. A comprehensive search for all available research on the topic was performed in three electronic bibliographic databases (MEDLINE in PubMed, PsycINFO, CINAHL) on 29th June 2015. No date limits were applied but search results were restricted to English. The search strategy included terms reflecting PA, PA disturbance, and sleep. PA search terms included variations of happy,
Results
From a total of 10,853 retrieved articles (4458 from PubMed, 4969 from PsycINFO, and 1426 from CINAHL), 161 titles and abstracts were identified as potentially relevant and full-texts were screened to determine eligibility (see Fig. 1). Forty-four articles fulfilled the inclusion criteria and were included for review.
Methodological challenges
This is the first systematic review to focus on the association between PA and sleep. Although findings from the studies reviewed support a link in healthy populations, as noted, a significant number of included studies showed weak methodological quality. Of primary concern is the limited number of longitudinal and experimental studies. Indeed, studies to date have largely been cross-sectional, making it difficult to infer the causal significance of associations. Overall, perhaps one of the
Future directions
First, limited research to date has focused on the potential stress-buffering or protective effects of PA [81], [82], [83]. As has been demonstrated in prior work, PA can influence stress in at least two ways. First, PA may indirectly influence sleep by modifying the effects of stress. In an illustrative study, Steptoe et al. [17] found that trait PA, as measured by ecological momentary assessment, partly accounted for the association between psychosocial risk factors (e.g., financial strains,
Conclusions
Although there is growing support for an association between PA and sleep, full understanding of the phenomenon is far from complete. The main issues limiting the validity and generalizability of the results include inadequate control of confounders, insufficient information regarding study design, small heterogeneous samples, and a paucity of longitudinal and experimental studies. More carefully conducted and theoretically informed research is needed before one can have confidence that PA
Conflict of interest
The authors do not have any conflicts of interest to disclose.
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