A Quick Guide to Mental Fatigue

A natural consequence of being engaged in your work, or of working longer hours, is a risk of mental fatigue. Mental fatigue is a very broad phenomenon: the brain is a very complicated organ, with many different working parts! Mental fatigue appears to be a warning sign that mental reserves are starting to become depleted. Its symptoms include increased task aversion and difficulty focusing attention on the task at hand.
Two contrasting examples:

• During the course of two hours of repetitive boring mental tasks, false-alarm error rates doubled from 5% to 10% and reaction time increased from 675 to 700 ms. Subjects became increasingly averse to continuing, and their tension level dropped. (Mental fatigue and task control: Planning and preparation)
  
• Subjects took part in a simulated workday. Reaction times decreased (got better!) during the morning and stabilized during the afternoon. For most people, error rates did not significantly increase throughout the day. However, subjects showed signs of subjective mental fatigue as the day progressed, especially if they had started with a high score on the Maslach Burnout Inventory. (The influence of mental fatigue on facial EMG activity during a simulated workday)

For a Swedish population:

"Fatigue was predicted by high work demands, low social support, not being a supervisor, female gender, lower age, lack of exercise, inability to stop thinking about work during leisure time (high immersion), snoring and disturbed sleep. The latter became the major predictor." (Mental fatigue, work and sleep)

People working 55+ hours per week do not in general seem to accumulate more mental fatigue due to their longer hours, unless they have high work demands and/or high immersion, in which case the combination seems to be quite nasty, and paying attention to mental fatigue issues becomes extremely important.

Obvious ways to combat mental fatigue include proper sleep (which also gives a boost to long-term happiness), physical exercise, work breaks, and task variation. Stress reduction is another obvious win, but how to best accomplish it is not well-understood; perhaps any activity that makes you feel more relaxed will do. The Mayo Clinic's list of suggestions is probably as good a starting point as any.