Fatigue resulting from sleep deficit can lead to decreased performance in a variety of cognitive domains and can result in potentially serious accidents. The present study aimed to test whether fatigue leads to increased Einstellung (low levels of cognitive flexibility) in a series of mathematical problem-solving tasks. Many situations involving fatigue and problem solving also involve people working in teams. However, little research has considered the role of social processes in managing the effects of fatigue. Research into the group monitoring hypothesis suggests that membership in a team can offset the effects of impairing factors such as fatigue upon performance. Thus, the present study also aimed to test whether group membership exacerbates or ameliorates the negative effects of fatigue. During the course of a weekend military training exercise, participants (N = 171) attempted to solve a series of problems either alone or in a team, and while either reasonably alert (nonfatigued) or fatigued through sleep deficit. Fatigued problem solvers working alone showed increased Einstellung. In contrast, and in line with the group monitoring hypothesis, teams of fatigued problem solvers did not experience increased Einstellung. The present study also showed that teams with a group member who was relatively less fatigued experienced less Einstellung than other groups. These effects persisted even once participants were cued toward more direct strategies. These findings highlight the risk of Einstellung when fatigued and also the importance of team membership with reference to problem solving in an occupational context. (Contains 2 tables, 1 figure and 1 footnote.)