  In the modern knowledge economy, success demands sustained focus and high
cognitive performance. Research suggests that human cognition is linked to a
finite resource, and upon its depletion, cognitive functions such as
self-control and decision-making may decline. While fatigue, among other
factors, affects human activity, how cognitive performance evolves during
extended periods of focus remains poorly understood. By analyzing performance
of a large cohort answering practice standardized test questions online, we
show that accuracy and learning decline as the test session progresses and
recover following prolonged breaks. To explain these findings, we hypothesize
that answering questions consumes some finite cognitive resources on which
performance depends, but these resources recover during breaks between test
questions. We propose a dynamic mechanism of the consumption and recovery of
these resources and show that it explains empirical findings and predicts
performance better than alternative hypotheses. While further controlled
experiments are needed to identify the physiological origin of these phenomena,
our work highlights the potential of empirical analysis of large-scale human
behavior data to explore cognitive behavior.
