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    The temporal association of caffeine and sleep in young adults : a thesis completed in partial fulfilment of the requirements for the degree of Master of Science in Nutrition and Dietetics at Massey University, Albany, New Zealand
    (Massey University, 2024) Suckling, Sarah
    Background: Young adults commonly experience poor sleep health due to various factors such as screen use, delaying bedtime, and high consumption of caffeine. Caffeine, a widely used psychostimulant, is known to negatively affect sleep quality by making it harder to fall asleep and reducing the time spent in REM sleep. Good sleep health is crucial for optimal mental and physical well-being and daytime performance. Despite young adults being one of the largest consumers of caffeinated products, particularly energy drinks, there's limited research on how this affects the sleep health of young adults, especially in New Zealand. Studies have revealed a bidirectional relationship between caffeine and sleep health, where reliance on caffeine during waking hours can lead to poorer sleep quality, creating a cycle of dependence. Aim: This study aimed to investigate the temporal associations between caffeine intake and sleep outcomes in young adults (18-25 years). This was investigated by assessing whether caffeine intake (dosage and timing) affected the subsequent nights sleep outcomes (duration, quality and timing), and whether sleep outcomes (duration, quality and timing) could affect the following days caffeine intake. Method: This study used GTX actigraphy and diary data to assess 7-days of sleep and caffeine intake information from 52 young adults (mean age: 22.06 + 2.043 years, 87% female). Diary data reported subjective sleep outcomes and caffeine intake and timing and actigraphy data was scored and analysed on computer software (Actilife, Version 6.1.2) then merged and aligned with diary data. Mixed linear regression models were used to analyse whether caffeine dosage and timing could predict sleep outcomes for the subsequent night’s sleep. Lagged effects were used to assess whether sleep duration, quality and timing could predict the following days caffeine intake. Both within person and between person variables were assessed. Results: Our results found as people consumed more than their personal average caffeine intake across the 7-days they slept longer, but, as people consumed more than the group average, their sleep was shorter. We also found that when someone slept longer than their personal average across the 7-days, they consumed more caffeine the next day, and when people slept longer than the group average they consumed more caffeine the following day. No association was found between caffeine timing and sleep outcomes, or caffeine intake on sleep efficiency and mid-point of sleep. Nor was any association found between sleep efficiency or mid-point of sleep on caffeine intake the following day. Conclusion: We found that caffeine consumption can adversely affect sleep duration, and sleep duration can predict the following days caffeine intake, creating a cyclic effect. Further research is required to determine how caffeine dosage and timing can impact other sleep variables such as quality and sleep timing. Due to the adverse effects that inadequate sleep duration can have on health and wellbeing and the extensive research on how caffeine intake can lead to shorter sleep duration, it would be valuable for remedies to be put in place to reduce caffeine intake in young adults.
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    Managing cabin crew fatigue during ultra-long range operations
    (Frontiers Media S A, 2023-12-19) van den Berg MJ; Zaslona JL; Muller DP; Wu L; Hughes M; Johnston B; Dyer C; Drane M; Signal TL; Fischer D
    Introduction: Ultra-long range (ULR) flights have the potential to increase fatigue-related risk for cabin crew, if the extended flight times are associated with extended wakefulness, sleep loss and higher levels of crew fatigue. ULR flights may also require longer opportunities for recovery sleep. This study evaluates the utilization of fatigue risk mitigations for cabin crew operating the Auckland – Chicago ULR route with a two-day layover. Methods: 65 cabin crew (45 women; aged 20–59 years) wore an actigraph and completed a sleep/duty diary for 3 local nights prior to, throughout, and for 3 local nights after a ULR trip. Crewmembers rated their fatigue (Samn-Perelli Crew Status Check), sleepiness (Karolinska Sleepiness Scale), and workload (OW; NASA-TLX) at key times during each flight. Jet lag was rated each day at home and during layover. Results: Fatigue and sleepiness were highest at top-of-descent and after landing and were higher on the inbound flight than on the outbound flight. For every hour of additional sleep in-flight, top-of-descent fatigue ratings decreased by 0.24 points and top-of-descent sleepiness ratings decreased by 0.38, whereas top-of-descent fatigue and sleepiness ratings increased by 0.24 points with every 10-point increase in OW ratings. Crew slept more in the 24-hours prior to the outbound (M= 8.5 h) and inbound flights (M= 9.1 h) compared to pre-trip baseline days (M= 8.2 h). Post-trip, crew slept more during the first day (M= 9.9 h) compared to baseline, with 95% taking a daytime nap. Jet lag ratings decreased daily on return home but were still higher on the fourth day than on the day of the outbound flight. Discussion: Cabin crew prepare for ULR flights by obtaining more sleep prior to departure. However, large individual differences in sleep and declining jet lag ratings across pre-trip days suggest that some crewmembers may still be recovering from a previous trip. Further refinement of in-flight sleep strategies and workload mitigations could be considered for managing fatigue risk at top-of-descent. Findings also highlight the importance of a protected period of post-trip rest to facilitate cabin crews’ recovery from the effects of sleep restriction and circadian disruption associated with this ULR trip.