Cognitive failures are errors in routine action regulation that increase with higher mental demands. In particular, in occupations where guidance such as teaching or supervision is essential, cognitive failures harm one’s performance and also negatively impact knowledge transfer. The aim of this study is to investigate yesterday’s work–home conflict (WHC) and objectively assessed sleep-onset latency as antecedents of a next-day increase in cognitive failures. Fifty-three teachers were assessed during a working week, in the morning, after work, and in the evening on each working day, as well as on Saturday morning. Sleep-onset latency was assessed with ambulatory actimetry. The multi-level analyses showed both WHC and sleep-onset latency predict cognitive failures the next working day (controlling for cognitive failures from the previous day, sleep quantity, and leisure time rumination until falling asleep). However, there was no association between yesterday’s WHCs and the nightly sleep-onset latency. Thus, nightly sleep-onset latency did not mediate the effects of yesterday’s WHCs on today’s cognitive failures. Our results highlight the importance of sleep and a good work–life balance for daily cognitive functioning. In order to promote the cognitive functioning of employees as well as occupational safety, good working conditions and recovery should both be considered.