Mental fatigue is known to occur as a result of activities related to e.g. transportation,
health-care, military operations and numerous other cognitive demanding tasks. Gaze
tracking has wide-ranging applications, with the technology becoming more compact and
processing power reducing. Though numerous techniques have been applied to measure
mental fatigue using gaze tracking, smooth-pursuit movement, a natural eye movement
generated when following a moving object with gaze, has not been explored with relation
to mental fatigue. In this paper, we report the results from a smooth-pursuit movement
based eye-typing experiment with varying task difficulty to generate cognitive load, performed
in the morning and afternoon by 36 participants. We have investigated the effects
of time-on-task and time of day on mental fatigue using self-reported questionnaires and
smooth-pursuit performance, extracted from the gaze data. The self-reported mental fatigue
increased due to time-on-task, but the time of day did not have an effect. The results
illustrate that smooth-pursuit movement performance declined with time-on-task, with
increased error in the gaze position and an inability to match the speed of the moving
object. The findings exhibit the feasibility of mental fatigue detection using smoothpursuit
movements during an eye-interactive task of eye-typing.