The present study set out to explore the locus of the poorly understood but frequently reported and comparatively large practice effect in sustained attention tests. Drawing on a recently proposed process model of sustained attention tests, several cognitive tasks were administered twice in order to examine which specific component of test performance benefitted from practice and to which extent. It was shown that the tasks representing the three sub-components of sustained attention tests, namely the perception of an item, the simple mental operation to solve an item, and the motor reaction to indicate a response to an item, benefitted from practice. Importantly, the largest practice gain was observed for the task that required item-solving processes in addition to perceptual and motor processes. Two additional postulated mechanisms in sustained attention tests--the deliberate shifting between items and the preprocessing of upcoming items--did not become more efficient through practice. Altogether, the present study shows that the practice effect in sustained attention tests seems to be primarily due to faster item-solving processes and, to a limited extent, due to a faster perception of the item, as well as a faster motor response. Moreover, besides the sub-components, it is likely that also the coordination of perceptual, item-solving, and motor processes benefitted from practice. Altogether, the present paper may have taken a first step towards a better understanding of the specific processes that cause the large practice gains in sustained attention tests.