Mental fatigue is a common but usually neglected phenomenon in built environments. It is a multidimensional phenomenon involving physiological, cognitive, motivational and mood states of the body, resulting in significant impairments of the individual’s mood, concentration and decision-making abilities.1 Mental fatigue usually arises from prolonged cognitive activities or sleep deprivation.2,3 It is usually followed by a reduction in working performance that is characterized by difficulties in initiating and maintaining performances on cognitive tasks. The typical symptoms of mental fatigue include slow responses, reduced eye movements, memory problems, concentration problems, increased risk-taking and reduced motivation.