In life, people often make choices between cognitive labor (high-demand tasks that produce external rewards) and cognitive leisure (low-demand tasks that provide relief from labor, but do not produce external rewards). For example, this happens when an employee decides between working on their spreadsheet vs. chatting with their colleague. In this talk, we examine how people make labor–leisure decisions. In particular, we focus on the role of mental fatigue, testing predictions derived from the opportunity cost model (Kurzban et al., 2013, Behavioural and Brain Sciences). In four lab studies, participants freely chose between labor (2-back task, paid) and leisure (e.g., playing with their smartphone). We consistently found that mental fatigue biased people’s choices towards leisure. Also, in line with the opportunity cost model, we found that the attractiveness of available leisure options was linked to mental fatigue—i.e., when people felt their leisure options were more attractive, they got fatigued more quickly during labor. Next, in two field studies, we examined the link between fatigue and naturally-occuring choices (specifically: how often people pick up their smartphone during work; how often people sit down and stand up during work). In sum, based on our research, we propose that the opportunity cost model is a promising tool for modeling free choices between labor and leisure in lab and life.