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 validation exercises support our Muslim-status classification, covering almost 11% of the sample and survey evidence indicates substantial Ramadan fasting compliance among Muslim chess players. In the preferred intention-to-treat specification, using pre-game controls, player fixed effects and year-month fixed effects, we find no impact of Ramadan fasting on Muslim players’ overall move quality or shares of optimal and nearly optimal moves, with tightly bounded estimates around zero. Muslim players make 0.13 additional percentage points of large errors during Ramadan, but this small estimate is fragile across alternative measures, samples, Muslim-status definitions, fasting-compliance adjustments and event-study diagnostics, with no evidence of heterogeneous effects, selection bias, or compensatory behavioural adjustments. We conclude there is little robust evidence that Ramadan fasting broadly impairs cognitive performance among expert chess players.

That is from a recently published paper by Samuel Buckland and David Smerdon.  Some claim that people think best when they are just a wee bit hungry?

Via the excellent Kevin Lewis.