Does fasting harm cognitive performance?

More than 2 billion people participate annually in Ramadan fasting, making its potential effects on cognitive performance important for workplaces, education and high-stakes decision-making. We study these effects in tournament chess, an incentivised, real-world cognitive task in which move quality can be evaluated objectively by a strong chess engine. We analyse nearly 300,000 games and more than 25 million moves played by almost 10,000 expert players from 178 countries over 10 years. Two valid...