Background: Student perceptions of teachers' interpersonal closeness positively affect their emotions. If closeness is, however, effortful for the teacher (i.e., emotional labour, signalling less genuine closeness), this may undermine these positive effects. We tested this assumption by using student reports and external observations of teacher closeness and ambulant measures of teacher heart rate, to gauge teachers' physiological effort connected to being close during class. Aims: We investigated the association between teachers' physiological effort connected to closeness and students' lesson-focused emotions. Sample: 75 teachers and their students (N = 1645) participated during one real-life lesson. Methods: Teacher interpersonal closeness was continuously coded based on a video recording and teachers' heart rate was measured continuously as an indicator of physiological effort. Students reported their emotions and perception of teacher closeness at the end of the lesson. Results: Multilevel models with student emotions as DVs and students' perceptions of teacher warmth (L1 predictor) and teachers' physiological effort when being close (i.e., an intra-individual cross-correlation, L2 predictor) were tested. As expected, students reported more positive and less negative emotions when they perceived more teacher closeness. The physiological effort connected to being close was not directly associated with student emotions; however, such effort moderated the effect of perceived closeness, especially with regard to negative student emotions (i.e., cross-level interactions). The more effortful teacher closeness was, the less closeness protected students from negative emotions. Conclusions: In line with extant research on faking enjoyment and emotional labour, students seemed to be affected when teacher closeness was physiologically effortful, and overall positive effects of teacher closeness were undermined.