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Many adolescents regularly use electronic devices, and a substantial proportion of adolescents use these devices in bed before they go to sleep. They are also at risk of using alcohol, recreational drugs, and tobacco, of becoming overweight or obese and having poor mental health. This thesis has investigated how adolescents' electronic device use affects their sleep and their wellbeing and examined (a) whether weekday and weekend sleep are explained using the same variables, (b) how device use affects sleep, and (c) how poor sleep affects mood. This thesis consists of three studies. Study 1 (N = 405) and study 2 (N = 489) both examined whether device use predicted poor and ill-timed sleep whilst simultaneously including other variables which are known to affect adolescents' sleep. Moreover, the studies also examined whether the factors which predicted weekday sleep also predicted weekend sleep. Thirdly, the studies examined the relationships between device use, sleep, and health related factors. The third study (N = 19) examined whether the blue light, content that was viewed on devices or the timing of device use affected adolescents' sleep and their following day mood. This thesis has shown that adolescents' weekday and weekend sleep are explained differently. Adolescents who were more anxious or were more aroused had poorer weekday sleep and adolescents who socialised more at the weekend had poorer weekend sleep. Importantly, the thesis has shown that more frequent device use, later use of electronic devices and using more interactive content predict poorer sleep quality and ill-timed sleep. Finally, this thesis has shown that poor and ill-timed sleep predicted worse mood. The findings from this thesis show that research should recognise that adolescents sleep differently on weekdays than they do at the weekend. The findings also show that researchers should investigate how the timing and content that individuals view on their electronic devices affects sleep.

Recent economic trends and wage stagnation are putting employees in challenging financial situations that may impact their contributions at work. This study draws on scarcity theory (Mullainathan & Shafir, 2013) to investigate how perceived financial scarcity and the experience of a scarcity mindset might impact early career professionals (i.e., graduate students) over a short period of time. A scarcity mindset is characterized by changes in people's thinking, such as increased attentional focus and neglect (i.e., tunneling), increased consideration of sacrifices (i.e., trade-off thinking), and increased cognitive load (i.e., the bandwidth tax; de Bruijn et al., 2021). These changes in cognition are associated with certain behaviors (i.e., borrowing and self-undermining) that create a self-perpetuating cycle of scarcity known as the scarcity trap. To test the model proposed by scarcity theory, the present study used a daily diary design and Multilevel Structural Equation modeling (MSEM). Doctoral students (N=93) reported perceived financial scarcity and changes in cognition and behavior via daily surveys. Across 19 days, participants reported 351 financial events, 211 of which were scarcity-inducing (i.e., shocks). The two most common shocks were related to food and housing, suggesting that the financial cost of meeting basic needs was enough to constitute a shock. When graduate students experienced a shock and an associated increase in their perceived early financial scarcity, they were more likely to make mistakes due to their cognitive resources being consumed by scarcity. Tunneling and trade-off thinking mediated the relation between perceived financial scarcity and financial borrowing. The bandwidth tax mediated the relation between perceived financial scarcity and self-undermining behavior. The existence of the scarcity trap was also partially supported by the reciprocal relationship between financial borrowing and future financial scarcity. These findings suggest that graduate students experiencing financial scarcity were more likely to borrow money and undermine themselves in their work (e.g. making mistakes). These behavioral changes may be attributed to the experience of a scarcity mindset through trade-off thinking, tunneling, and the bandwidth tax. In sum, this study offers support for scarcity theory as an explanatory mechanism for short-term changes in employee thinking and behavior that may perpetuate financial scarcity and its negative effects in the long run. Implications for research and practice are discussed.

Drawing upon the broaden-and-build theory of positive emotions (Fredrickson, 2001), this study examines the links between morning physical activity, subsequent positive emotions, broadened thinking, and psychological resource accumulation at work. Fifty-two participants who worked full-time completed a daily diary for 10 workdays that included measures of their emotions and physical activity each morning and measures of broadened thinking each afternoon. Psychological resources were assessed at the beginning and end of the 10-day period. Data were analyzed using multilevel structural equation modeling (MSEM) with Mplus. Results did not support the mediating role of positive emotions and broadened thinking between morning physical activity and psychological resources, as would be predicted by the broaden-and-build theory. However, results did indicate that participants experienced more positive emotions on the days they engaged in morning physical activity. Additionally, participants who more frequently engaged in morning physical activity experienced positive emotions more often during the study period. There was also evidence to suggest that positive emotions promoted planning and active coping in response to workplace problems. In sum, these findings indicate morning physical activity may boost employees' emotional states and experiences of positive emotions may result in the use of broad-minded coping strategies at work. Implications for research and practice are discussed.