Sleep deprivation affects mental performance, as anyone who has tried to work after an all-nighter can attest. Yet some professionals, including surgeons, firefighters and military personnel, must routinely work on little or no sleep. A study by researchers at the University of Texas at Austin found the sleepy brain’s Achilles’ heel—open-ended problem solving—and thus may help improve worker training in these demanding fields.

The study, which was published in Sleep in March, consisted of two types of learning tests. In the first test, sleep-deprived students were asked to cate­gorize drawings of fictional animals as either “A” or “not A,” an open-ended task that depended on the students’ ability to remember criteria for “A” and apply it consistently. In the second test, the students sorted two types of fic­tional animals, “A” and “B.” The second test was more complex in that it re­quired students to learn criteria for two animals instead of one, but surprisingly, sleep deprivation had the largest effect on the first test.

The researchers suspect that attention lapses—one of the main consequences of sleep loss—are to blame. Previous studies suggest that open-ended tasks, such as the first test, require more focused attention than those that offer two clear choices, as the second test did. “When we get sleep-deprived, some of our brain’s learning systems operate better than others,” notes Todd Maddox, the study’s lead author. Fortunately, Maddox says, the more we know about the sleep-deprived brain, the better we can train people to work around its shortcomings.