The present study investigated psychological capital (PsyCap) as a protective factor in the relationship between worries about accidents and sleepiness among seafarers. The hypothesis that strong PsyCap weakens the relationship between worries about accidents and sleepiness was tested in a cross-sectional sample of 397 maritime workers. In contrast to expectations, the findings indicated a reverse buffering effect in that PsyCap only had a protective impact on sleepiness when worries about accidents were low. For workers that were highly worried, a strong PsyCap was associated with increased levels of sleepiness. The established associations remained consistent after controlling for workers’ years of experience as seafarers, and their ratings of psychological safety climate. An interpretation of this finding is that seafarers with high levels of PsyCap will be attentive when the threat level is serious, but will not be bothered when exposed to everyday strain and hassles associated with their work situation.