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Although seafaring is one of the most important professions for trade in goods in the 21st century, the results of recent research indicate that seafarers face various sources of stress in their work, including high workload and long working hours. These stressors can be reflected in low job satisfaction and poor mental health, which can result in sleep problems, and consequently lead to various somatic symptoms. The main goal of this mixed method research was, therefore, to expand knowledge about seafarers' sleep on board, whereby a sequential explanatory design was used. Study 1 (N = 286 male seafarers from Croatia) examined whether the number of sleep hours on board has a mediating role in explaining the relationship between job satisfaction and mental health as predictors of somatic symptoms. The results of the path analysis showed that higher job satisfaction directly and indirectly through longer sleep on board reduces the number of somatic symptoms, while better mental health directly contributes to fewer somatic symptoms. Study 2 (N = 205 seafarers from Croatia) aimed to get a deeper insight into the sleep of seafarers on board, whereby 39% (N = 80) of the participants had difficulties related to sleep problems on board. The thematic analysis of their answers (N = 76) showed the sleeping difficulties experienced by seafarers on board and gave an insight into the consequences of sleeping difficulties and the coping strategies they use. The results of this research indicate the importance of developing healthy work environments for seafarers.