Using test data for all children attending Danish public schools betweenschool years 2009/10 and 2012/13, we examine how the time of the testaffects performance. Test time is determined by the weekly classschedule and computer availability at the school. We find that, forevery hour later in the day, test performance decreases by 0.9% of an SD(95% CI, 0.7-1.0%). However, a 20- to 30-minute break improves averagetest performance by 1.7% of an SD (95% CI, 1.2-2.2%). These findingshave two important policy implications: First, cognitive fatigue shouldbe taken into consideration when deciding on the length of the schoolday and the frequency and duration of breaks throughout the day. Second,school accountability systems should control for the influence ofexternal factors on test scores.