## 4 Discussion

Table 7: _Cont._Adverse environmental conditions, such as exposure to noise, vibrations and heat stress, have been found to increase the risk of sleep disturbances [5; 21; 22]. Noise exposure has adverse effects on the quantity and quality of sleep [21; 22], but cumulative exposure to vibrations may cause a synergistic effect increasing sleep disruptions [21]. In our study, the perception of high levels of discomfort regarding the exposure to vibrations and noise was higher in the group working on the early morning shift, followed by those on the night shift. Regarding heat stress, exposure to this environmental risk factor may increase the risk of sleep disturbances [5]. Environmental risk factors may contribute to a higher probability of increased sleep disturbances in the groups working during the night and early morning shifts, but these variables did not appear as predictors in the regression model. Psychosocial factors are related to the quality and quantity of sleep, and several studies show associations of job control, social support and meaning of work with quality or quantity of sleep, although some findings might be inconsistent [4; 23; 24]. The cognitive demands are mainly related to memory requests, the need for problem solving and giving new ideas during work activity [24]. In both data collection moments, these seemed to be more relevant during fixed daytime shifts. When these demands are too high, the negative impact on stress perception contributes to a disrupted sleep [4; 23; 24]. The meaning of work showed differences between those on permanent night and early morning shifts, with the best values were seen among the night shift group, in 2017. These differences did not appear in 2019. A meaningful working life is a relevant variable positively influencing sleep quality [23]. Regarding horizontal trust in 2017, worse results were perceived by those on the night shift. These workers do not meet other groups from other departments because they work permanently from 24:00 till 8:00. This might influence their perception of trust or support from other departments or divisions. Social support contributes to a reduction in sleep troubles [23], playing a relevant role in the management of the impact of shift work. Social support may act as a buffer to high work demands or low degree of control, protecting against the effects of night shift work [4]. In 2019, job satisfaction showed negative results within the group working the early morning shift. Different activity sectors showed an association between job satisfaction and sleep quality, which was more evident in daytime shifts [25]. Actually, the relationship between job satisfaction and sleep quality may depend on the shift, considering that workers on night and early morning shifts are more used to poor sleep quality [25]. Among dissatisfied workers, an increase in stress levels may negatively influence health and sleep [26]. From 2017 to 2019, SMAS faced organizational changes with a strategy that emphasized the acquisition of new pieces of equipment for daytime shifts, which may have impacted the overall perception of rewards and recognition in some areas. This may explain the rising dissatisfaction among those on the early morning shift, but the interpretation must be cautious because satisfaction is a multifactorial construct. It is well established that environmental exposures and physical and psychosocial work factors influence the quality and quantity of sleep. In the present study, sleep disturbances were perceived more negatively by those working permanent night and early morning shifts. The main differences were identified between those two types of shifts and the rotating and fixed daytime shifts, meaning that different types and fixed shifts have a distinct impact on sleep disturbances. These findings are in agreement with the literature stressing that night shifts are more problematic in relation to sleep disturbances [2; 3; 8].