Mental fatigue

Boksem and Tops proposed a framework in which people keep weighing the expected rewards of a task against its energy costs [2]. In their account, people stop being motivated to keep performing when the costs are perceived to outweigh the predicted rewards [2]. On mechanism, their review points to brain regions including the nucleus accumbens, orbitofrontal cortex, amygdala, insula and anterior cingulate cortex as involved in that weighing [2]. It is a proposed framework, not a settled explanation.

Mental fatigue also shows up in physical performance research. A systematic review of 11 studies found that mental fatigue was associated with poorer endurance performance, such as a shorter time to exhaustion or a longer completion time, together with higher than normal perceived exertion [1]. Heart rate, blood lactate and oxygen uptake were unaffected [1]. Maximal strength, power and anaerobic work were also unaffected [1].

Because mental fatigue follows prolonged demanding cognitive activity, a study of it depends on the fatiguing task it used [1]. The review above included studies that tested whether mental fatigue changed performance on a physical task, and it excluded studies that used self-regulatory depletion tasks shorter than 30 minutes [1]. When a trial reports mental fatigue, check how long and how demanding the fatiguing task was, and whether both groups did the same fatiguing task.

On NootropicsWork, mental fatigue is one of the outcomes marked as subjective. When a trial's result rests on participants' own ratings, blinding matters more: check whether participants could tell which group they were in. Mental fatigue is also kept apart from daytime fatigue, a separate outcome linked to sleep, and from subjective energy, which is a separate outcome.

Cognitive load, vigilance and attention.