Male shift workers and cardio-metabolic function: The role of homeostatic desynchronisation and exercise as an intervention

Shift work is a work structure designed to increase total labour opportunity and meet the growing demands of modern society. However, the extended and rotating labour periods are associated with adverse health conditions including an increased risk of developing cardio-metabolic disorders. Given the integrated nature of cardio-metabolic regulation, the biologically disruptive labour characteristics of shift work, including night and rotating shifts, may facilitate disease progression. Additionally, given the central role of shift work within the modern workforce, developing interventions to improve health outcomes have become a research focus. Exercise is a thoroughly researched and validated intervention to improve the general health of a variety of population groups. However, limited research currently substantiates these observations among shift workers, a population group with established barriers to exercise participation. Therefore, the aims of the current thesis are to i) examine the effect of employment in rotational shift work on markers of cardio-metabolic function and ii) examine the effect of exercise, including variations in mode, intensity, and chronicity (acute vs training), on markers of cardio-metabolic function among rotational shift workers.

The first study in this thesis aimed to investigate the effect of rotational shift work on measures of cardio-metabolic function including sleep, inflammatory status, glucose metabolism and insulin sensitivity, body composition and autonomic regulation. Eighty-seven sedentary but healthy men, matched for lifestyle behaviours including smoking status, average sleep quality and physical activity (PA) status were categorised via occupation; shift workers (n = 44) and non-shift workers (n = 43). The shift worker group reported significantly reduced aerobic capacity (VO2peak), increased response time to an oral glucose tolerance test, higher body fat, and higher resting values of interleukin (IL)-6. Conclusively, shift work is associated with an increase in independent risk factors for cardio-metabolic disorders, suggesting a detrimental effect of shift on the health and well-being of male employees.
The second study in this thesis explored the acute effect of moderate continuous (MICT) and high intensity interval training (HIT) on markers of cardio-metabolic function and sleep among rotational shift workers. Twenty-six sedentary men currently employed in rotational shift work underwent baseline laboratory testing (including 7 days sleep assessment) before being randomly allocated a 30 min cycling intervention of either HIT: 1:4 ratio of 60 s at 100 % and 240 s at 50 % VO2peak, or MICT; continuous cycling at 60 % VO2peak. Laboratory testing was recorded post intervention (immediately, 30- and 60-minutes post) before subsequent sleep opportunity was assessed. A significant increase in IL-1 receptor agonist (IL-Ra) was observed immediately and 30 minutes post HIT but not MICT. Alternatively, MICT significantly reduced wake after sleep onset (WASO) in the subsequent sleep opportunity, a result not observed within the HIT group. Collectively both HIT and MICT acutely improved markers and modulators of cardio-metabolic function and warrant further investigation into the potential intensity dependant adaptations.

The final study in this thesis investigated the effect of a 12-week training intervention of MICT or resistance training (RT) among thirty-eight sedentary male shift workers. Randomly assigned a control, MICT or RT group, participants underwent baseline laboratory testing before completing 12 weeks of training, prescribed 3 days a week. Mean sessional attendance across the intervention was 25 (± 7) of a possible 36 sessions despite the provision of free, personalised and semi-supervised training. A significant effect was observed among the MICT group, reducing c-reactive protein (CRP) levels post intervention. Both the MICT and RT increased total sleep time (TST) following a night shift post-intervention. Finally, data was redistributed based on sessional attendance to investigate a potential dose-response effect. A significant reduction in body fat was observed when 24 or more total sessions were attended during the intervention. The results suggest exercise is a valid intervention to improve the cardio-metabolic function of male rotational shift workers however several barriers to exercise adherence need to be investigated.
Collectively, the thesis contributes to the body of research investigating the increased risk of cardio-metabolic disorders among male rotational shift workers by exploring the effect of homeostatic desynchronisation on markers of cardio-metabolic function. Further, this research contributes to the currently limited body of exercise-based intervention research among rotational shift workers. Both acute and chronic training interventions improved measures commonly associated with poor cardio-metabolic function and future outcomes. However, further research is required to investigate intensity and mode-based adaptations as well as adherence factors.