PhD students come to work in academic environments that are characterized by long working hours and work done on non-standard hours due to increasing job demands and metric evaluation systems. Yet their long working hours and work at non-standard hours are often seen as a logical consequence of their intellectual quest and academic calling and may even serve as a proxy for their research engagement. Against that background, quantitative data from 514 PhD students were used to unravel the complex relationships between different aspects of time use and PhD students' work engagement. While the results support the academia as a calling thesis to some extent, they also show that the relationships between long and non-standard working hours and research engagement are partly negated by the fact that the same working time characteristics lead to perceived time pressure and lack of time sovereignty, which in turn negatively affects their engagement. Moreover, the mechanism behind this negation varies across scientific disciplines. These subjective working time characteristics are the same alarm signals that are flagged as risk factors in academic staff for occupational stress, burnout, and work-life imbalance and thus cannot be ignored.