This report presents the findings of Project Horizon – a European Commission part-funded multi-partner research initiative to investigate the impact of watchkeeping 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 watchkeepers 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. Project Horizon is a major multi-partner European research study that brought together 11 academic institutions and shipping industry organisations with the agreed aim of delivering empirical data to provide a better understanding of the way in which watchkeeping patterns can affect ships’ watchkeepers. The broad spread of the project partners ensured expert objectivity of the project and its results, as well as widening routes for dissemination and exploitation of the findings.