Post-lunch impairments in performance efficiency have been found in both real-life settings and the laboratory. The first aim of the present experiment was to determine whether post-lunch changes in performance depend on eating a meal, or whether they are also found in subjects who abstain from eating. A second aim was to examine which aspects of performance are most likely to show post-lunch impairments. The results showed that the ability to maintain attention and react quickly to the appearance of a new stimulus was impaired following consumption of the meal. The extent of this effect did not vary with time on task, and was unaltered by changes in the time at which lunch was eaten. In contrast to this, the ability to resist distraction was unaffected by eating lunch. Other types of performance, such as movement time, varied as a function of time of day, but were not impaired by the meal.