Social jetlag refers to the chronic shift in sleep timing between work and free days and has been associated with a variety of negative physical and psychological outcomes. Existing research on social jetlag has relied heavily on cross-sectional studies, preventing researchers and practitioners from assessing the effects of social jetlag over time. The current study used longitudinal data to explore the directionality of the association between social jetlag and depressive symptoms as well as the longitudinal associations between social jetlag, academic performance, and wellbeing in college students. Gender and race were also assessed as potential moderators of these associations. Cross-lagged panel analysis using Amos for SPSS revealed that social jetlag predicted depressive symptoms both concurrently adjusting for covariates and longitudinally in unadjusted models. This finding suggests that sleep disturbances may precede mood concerns, although causality cannot be proven due to the design of the current study. PROCESS moderation analyses indicated that social jetlag did not significantly predict academic performance or wellbeing over time, and neither gender nor race moderated these associations. Future research is needed to further assess the short- and long-term outcomes of social jetlag using prospective, well-controlled studies and objective measures of sleep timing.

Midlife women are vulnerable to developing obesity. Both sleep timing and negative emotion are risk factors, yet limited research has explored their role on weight outcomes in this population. The current investigation explored the association of sleep timing (i.e., mean sleep time, sleep time variability) and obesity (i.e., BMI, waist circumference) in midlife women, considering emotional pathways (i.e., depressive symptoms, anger) that might mediate this association. PROCESS parallel mediation models assessed direct and indirect pathways. In follow-up analyses, sleep duration was explored as an indirect pathway linking sleep timing to obesity. Results demonstrated that sleep timing does not directly predict obesity. Emotion was also not a significant indirect pathway. Conversely, sleep duration emerged as an indirect link in the sleep timing and obesity association. Future work is needed to further disentangle the impact of sleep on weight in midlife women using prospective, well-controlled studies, implementing daily assessments of sleep behavior.

The circadian clock defines physiologically optimal times for sleeping, which vary along a continuum of circadian phenotypes from morning- to evening-type. Although different ‘chronotypes’ can be discriminated reliably by the Morningness/Eveningness Questionnaire (MEQ), there is little published information on their prevalence. The timing of sleep is also heavily influenced by societal norms. However, the relative contribution of circadian physiology versus psychosocial factors is unknown. This thesis took a multidimensional approach to investigating preferred sleep timing within the general population of New Zealand (30-49 years). A New Zealand version of the MEQ was mailed to a random stratified sample of 5,000 adults living in the Wellington region (55.7% response rate). Using scoring criteria for middle-aged adults, approximately 25% of the population were morning-types and 25% were evening-types. The sleeping patterns of 15 morning- and 16 evening-types were monitored using actiwatches and sleep diaries. Morning-types slept significantly earlier, but there were no differences in sleep duration or quality. Both chronotypes showed evidence of using the weekend to catch-up on sleep, although this was more evident among evening-types. Differences between chronotypes were also investigated using the endogenous melatonin rhythm as a circadian phase marker. The timing of the melatonin rhythm was earlier among morning-types, with the difference being greater for melatonin onset, than offset. However, differences between weekday versus weekend sleep explained more of the variability in sleep timing that did circadian phase. Understanding the genetic differences in the circadian clock is evolving rapidly. While this is of particular scientific interest, little consideration has been given to the ethical implications of this type of work. In the final study, a Kaupapa Māori framework was used to explore Māori hopes and concerns for genetic research in Aotearoa/New Zealand. Thematic analysis indicated that Māori are not anti-science, however there is an urgent need for ethical guidelines that uphold and respect the values of Māori society. This thesis argues that sleep is a major public health issue for New Zealand. However, a number of challenges must be met to ensure that new scientific knowledge meets the needs and expectations of the community.

The main objective of this thesis was to examine the independent effects of sleep duration and timing on appetite, food reward and energy balance. Study 1 investigated the associations between satiety quotient (SQ) with habitual, self-reported sleep duration, quality and timing. No significant associations were noted between SQ and sleep parameters. Short-duration sleepers had a lower mean SQ vs. those with ≥7h sleep/night (P=0.04). Study 2 evaluated associations between changes in sleep duration, efficiency and timing with changes in next day food reward. Greater sleep duration and earlier wake-times were associated with greater food reward (P=0.001). However, these associations were no longer significant after controlling for elapsed time between awakening and completion of the food reward task. Study 3 examined the effects of 50% sleep restriction (SR) anchored during the first (delayed bedtime) or second (advanced wake-time) half of the night on appetite, SQ, food reward, energy intake (EI) and energy expenditure (EE). Greater appetite ratings and explicit high-fat food reward were noted following SR with an advanced wake-time vs. control and SR with a delayed bedtime (P=0.03-0.01). No difference in SQ was noted between sessions. Energy and carbohydrate intakes were greater on day 2 and over 36h in the delayed bedtime vs. control session (P=0.03). Activity EE and moderate-intensity physical activity (PA) time were greater following delayed bedtime vs. control and advanced wake-time on day 1, whereas vigorous-intensity PA time was greater following advanced wake-time vs. delayed bedtime on day 1 (P=0.01-0.04). Greater sleep quality and slow-wave sleep duration between SR sessions were associated with lower EI and increased vigorous-intensity PA time, respectively (P=0.01-0.04). Collectively, these findings suggest that appetite, SQ and food reward are influenced by sleep parameters, but these changes may not alter EI. These findings also suggest that individuals with greater sleep quality in response to SR had greater vigorous-intensity activity time and lower EI.