## 1 Introduction  
The development of cities with a growing production of waste and increased use of water and sanitation systems has been pressuring the water, sanitation and waste sectors managed by municipalities in Portugal. Municipalities manage the collection and haulage of waste and perform the maintenance of water and sanitation systems related to household and economic and public activities, such as restaurants, schools and companies [1]. Municipalities have blue-collar workers devoted to regular waste collection and the maintenance of the water and sanitation systems. These tasks are mainly physically demanding, but, in the last few decades, the implementation of organizational and technological changes aimed to improve the efficiency and sustainability of the water, sanitation and waste sector [1]. These services offered to all urban and country populations often implicate a 24 h work activity, with shift and night work. The exposure of workers to shift work and night work has well-described consequences, such as the perturbation of sleep and wake cycles and the modification of the activity and rest pattern [2]. Frequent consequences of shift and night work are sleepiness and fatigue during work periods, health impairments and increased safety risks [2, 3]. In addition, work conditions may vary among different shift types, depending on the nature of the tasks, the number of workers, the work pace, the presence of supervisors, and other factors [3; 4]. Physical and psychosocial characteristics of work influence workers' sleep and may have a cumulative effect on shift work. Higher job demands, such as quantitative demands, role conflict, decision control and social support [5; 6; 7], and environmental exposure to noise or heat stress are related to poor sleep quality [5], and high physical workload is associated with sleep problems and fatigue [3]. In the literature, night shifts relate to more negative effects on health and social relations [8], with workers facing more difficulties coping with and leaving these types of shifts earlier in work life [9].  
Generally, municipal workers belong to different occupational categories with diverse activity demands, and are an ageing population [10]. Those belonging to the water, sanitation and waste sectors are less focused on research but exposed to high physically demanding tasks, outdoor work, shift work, and psychosocial risk factors [11]. Still, there is a gap in research regarding the differences in the perception of working conditions between distinct shifts among workers from the water, sanitation, and waste sectors and the consequences for sleep disturbance.  
The present study aimed to characterize the perception of the main determinants of work activity among workers from the water, sanitation, and waste sectors in Portugal, in 2017 and 2019, analyze the variations in the work demands among different types of shifts, and identify the main predictors of sleep disturbance in 2017 and 2019.  
An ageing population was identified in all shift types. Possible occupational trajectories with changes from the permanent night and early morning shifts to daytime and fast rotating shifts may be linked to health conditions. Workers in fixed night and early morning shifts perceived higher physical demands and environmental discomfort, lower social support, and job dissatisfaction. Workers in daytime or fast rotating shifts perceived higher cognitive demands. Sleep disturbances were perceived more negatively among those working permanently on night and early morning shifts in both data collection moments. The main predictors of sleep disturbance in 2017 and 2019 were the type of shift, namely, night and early morning shifts, and high physical demands.