## 4 Discussion
Contrastingly, some studies focusing on permanent night shifts point to an improvement in individual adaptability [26]. This may be the case for some workers on night shifts, considering the average ages and the higher seniority (>15 years) of these workers (24%). In modern societies, sleep problems are common in occupational groups, with negative consequences for health [27]. No sleep disturbances between 50 and 70 years of age is associated with a long, healthy life [27]. So, identifying the main predictors of sleep disturbances is relevant to defining specific measures that prevent negative effects on the health and well-being of these workers. It is also relevant to identify the profiles of susceptible workers that will benefit from support strategies offered by the organization. Ourregression models reveal that, in 2017 and 2019, shift type was one of the main predictors of sleep disturbances: working in permanent night or early morning shifts was the main determinant of sleep disturbances, which is in accordance with the literature [2, 28, 29, 30]. The presence of high physical demands, such as the adoption of awkward postures, was a predictor of sleep disturbances. Some authors related intense physical work and tiring postures with the inception of sleep disturbances [6, 18]. In 2017, work seniority, horizontal trust and auto-efficacy were protective factors contributing to decreasing sleep disturbances. The role of work seniority can be understood from two perspectives: regarding the possible ability of some of these workers to adapt to permanent night shifts [14]; considering the possible pathways in working trajectories from permanent night shifts to day shifts or rotating ones. Horizontal trust, meaning that these workers trust co-workers, studied from the perspective of commitment, has a positive impact on retaining workers in organizations [30]. In our model, horizontal trust is related to lower sleep disturbances. This positive effect can emerge from increased group cohesion resulting from better trust [30] and social support, which prevents sleep troubles [23]. Auto-efficacy also appeared as a protective factor. It is related to the belief in the ability to perform actions required in certain situations, and is a key factor in engagement in effortful tasks [31]. Although there is no consensus regarding the influence of auto-efficacy and personality-related variables on shift work tolerance [14], some studies showed a positive relationship between internal locus of control and shiftwork tolerance [32]. In 2019, these protective factors did not appear in the model, while the participants were the same. Organizational changes that occurred in the period between the two data collection moments may have influenced the perception of workers of horizontal trust and social support from colleagues. Some limitations of this study are related to the different tasks performed in the two divisions, involving different levels of physical or mental demands, possibly acting as confounders concerning shiftwork effects, and making comparisons more difficult. Another limitation results from the self-reported questionnaires and the possibility of recall bias. Recall bias is very plausible in the variable of seniority and others. Furthermore, a self-selection process is typical in shift work, and is likely to result in a healthy worker effect [14]. Finally, the proportion of the total variance of sleep disturbance explained by the multiple linear regression models was low in both data collection moments, which indicates that the variables considered in the study did not explain a substantial part of the total variance of the sleep disturbance.