This report presents the findings of Project Horizon – a European Commission part-funded multi-partner research initiative to investigate the impact of watch-keeping patterns on the cognitive performance of seafarers. This pioneering research sought to advance understanding of seafarer fatigue through scientific analysis of data drawn from realistic working scenarios using experienced watch-keepers on ship simulators. The report explains the reasons why the project was considered necessary and how the research was undertaken, as well as presenting the findings and research outcomes. The project has taken knowledge in this area to a new level, demonstrating conclusively the links between performance degradation and certain patterns of work. The project surpasses previous subjective fatigue studies, delivering validated, scientifically and statistically robust results that can be used to help determine safer working patterns in the interests of the safety of life at sea, the safety and security of the marine transport system and the protection of the marine environment.