The Hour Between Dog and Wolf: How Risk Taking Transforms Us, Body and Mind

by John Coates

A telling example can be found in our understanding of mental fatigue. Common sense tells us that it is a state of exhaustion, in which we have quite simply run out of energy, like a car running out of gas. The recommendation that naturally follows is a rest or vacation to replenish our energy reserves. Exhaustion of this kind certainly occurs. Run a marathon, and chances are you end up in a state of exhaustion; pull an all-nighter, and chances are you need some sleep. But more often than not, this is not the cause of mental fatigue. Often, mental fatigue disappears if we merely change activities, and that would not happen if we had exhausted our fuel. A recently developed model in neuroscience provides an alternative explanation of fatigue. According to this model, fatigue should be understood as a signal our body and brain use to inform us that the expected return from our current activity has dropped below its metabolic cost. The brain quietly searches for the optimal allocation of attentional and metabolic resources, and fatigue is one way it communicates its results. If we are engaged in some form of search and have not turned up any results, our brain, through the language of fatigue and distractibility, tells us we are wasting our time and encourages us to look elsewhere. The cure for fatigue, according to this account, is not a rest; it is a fresh task. Support for this idea comes from data showing that overtime work does not in itself lead to work-related illness ...