I asked this question a long while ago, but I never got an answer to whether mental exhaustion can be measured. If mental exhaustion could be measured, it would mean that I would have a way of knowing the performance I can get out of my brain in a day. I would be able to know what is best for refilling my brain with energy (maybe TV is your best option after all...;) ) As far as I understand, different parts of the brain gets used for different purposes, so they should get exhausted at different times, so the measuring would have to be on specific regions of the brain or performance of the brain in various fields.
A sleep study by Van Dongen et al (2003) uses three metrics to measure the effects of fatigue due to sleep loss. The first is cognitive throughput:
A serial addition/subtraction task was included in the assessment battery to measure cognitive throughput. The serial addition/subtraction task is a subject-paced task requiring the completion of 50 mental arithmetic trials. The average number of correct responses per min was used as a neurobehavioral assay of cognitive throughput performance.
Second, alertness / vigilance (response time to stimuli):
The neurobehavioral assessment battery included a psychomotor vigilance task to measure behavioral alertness. The psychomotor vigilance task (PVT) is a sustained-attention reaction time task with a random inter-stimulus interval of 2–10 s. Lapses (reaction times greater than 500 ms) were counted per 10 min test bout as a measure of performance.
The neurobehavioral assessment battery also included a computerized digit symbol substitution task to measure working memory. This subject-paced task involves the matching of digits (0–9) to symbols (circle, triangle, etc.). The number of correct responses in 1.5 min was counted to measure working memory performance.
The metrics ought to be effective for measuring mental performance due to fatigue whatever the cause of fatigue might be.
- Van Dongen, H.P.A., Maislin, G., Mullington, J.M., Dinges, D.F. & others (2003). The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. SLEEP-NEW YORK THEN WESTCHESTER-, 26, 117-129.
There are a several perspectives of mental fatigue. Here are a few starting points:
- There is a huge literature on burnout (see wikipedia article for a review, and Maslach et al, 2001, PDF) for their annual review of psychology article. Burnout refers to a rather chronic form of emotional and mental exhaustion. Burnout is often assessed in research using questionnaires, but obviously it could be clinically assessed. Of course, your question seems more concerned with short term fatigue rather than longer term fatigue, and cognitive fatigue rather than emotional fatigue.
- There are a lot of studies looking at fatigue and work hours (see this google scholar search). Such studies are often conducted within the context of human factors, occupational health, and work stress perspectives. You might like this one by Folkard et al (2003, PDF) on shift work, safety and productivity, it has some graphs relating time of day and time since break to productivity (i.e., not exactly answering your question, but marginally relevant).
- You could look at the literature on vigilance that discusses the general deterioration of vigilance skills over time (see this wikipedia discussion).