This thesis examines the impact of Ramadan fasting on cognitive performance by analysing the quality of moves made by professional chess players during tournaments. Using a dataset of nearly 300,000 games played over ten years by expert players from 178 countries, we investigate whether fasting during Ramadan affects the quality of chess moves, a proxy for cognitive function. Employing fixed-effects regression models, we find no statistically significant impact of Ramadan fasting on the quality of moves played by Muslim chess players. These findings are robust across different measures of move quality, samples, methods of identifying Muslim players, and regression specifications. We exploit the large sample size to demonstrate that the estimated effects are economically insignificant rather than simply reflecting statistical noise. We also explore heterogeneity of the effects and potential mechanisms behind the null results, such as selection bias and offsetting positive effects during Ramadan, but find no evidence supporting these explanations. Our study is the first to assess the impact of Ramadan fasting on cognitive performance using professional chess as a cognitive task, providing a comprehensive analysis across a wide age range and a diverse international sample. By extending the examination beyond educational settings to a high-stakes competitive environment, we offer novel insights into how fasting may influence complex cognitive functions. The findings suggest that, within the context of chess, Ramadan fasting does not impair cognitive function.