New research suggests that the brain's "executive function," the mental system involved in abstract thinking, planning, and focusing on one thing instead of another, can be fatigued to the point that your ability to make decisions is badly hampered. Scientific American has an interesting survey of studies in this area. The, er, decision whether to read the article or not is up to you. From SciAm:
...What types of actions exhaust executive function and affect subsequent decision-making? Until recently, researchers focused on activities that involved the exertion of self-control or the regulation of attention. For instance, it's long been recognized that strenuous cognitive tasks–such as taking the SAT–can make it harder to focus later on. But recent results suggests that these taxing mental activities may be much broader in scope-and may even involve the very common activity of making choices itself. In a series of experiments and field studies, University of Minnesota psychologist Kathleen Vohs and colleagues repeatedly demonstrate that the mere act of making a selection may deplete executive resources. For example, in one study the researchers found that participants who made more choices in a mall were less likely to persist and do well in solving simple algebra problems. In another task in the same study, students who had to mark preferences about the courses they would take to satisfy their degree requirements were much more likely to procrastinate on preparing for an important test. Instead of studying, these "tired" minds engaged in distracting leisure activities.
Why is making a determination so taxing? Evidence implicates two important components: commitment and tradeoff resolution. The first is predicated on the notion that committing to a given course requires switching from a state of deliberation to one of implementation. In other words, you have to make a transition from thinking about options to actually following through on a decision. This switch, according to Vohs, requires executive resources. In a parallel investigation, Yale University professor Nathan Novemsky and his colleagues suggest that the mere act of resolving tradeoffs may be depleting. For example, in one study, the scientists show that people who had to rate the attractiveness of different options were much less depleted than those who had to actually make choices between the very same options.
Tough Choices: How Making Decisions Tires Your Brain (Scientific American)