The influx of communication media to contemporary workplaces has exposed workers to be always connected to their work. Constant connectivity to work (CCW) refers to the condition in which workers are always connected to work, even during their non-work hours, and are not detached from the work situation. We investigated the association between CCW and insomnia and the moderating effect of work engagement. A total of 29,512 nationally representative samples of workers in Korea were used. Insomnia was assessed by the Minimal Insomnia Symptom Scale. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs). The additive interaction between CCW and low work engagement was estimated by calculating Relative Excess Risk due to Interaction (RERI). Exposure to CCW was related to insomnia (OR [95% CI] 1.33 [1.22-1.46]). Additionally, a significant negative interaction between CCW and work engagement was observed. The effects of CCW were mitigated in those with high work engagement. The OR of the combined effect of CCW exposure and low work engagement was 2.52 (95% CI 2.22-2.87). RERI between exposure to CCW and low work engagement was 0.69 (95% CI 0.38-0.99), indicating that there is a supra-additive interaction. Our study found that CCW is related to an increased risk of insomnia and that high work engagement can mitigate the effect of CCW. Our study suggests that improving work engagement and disconnecting from work outside of work hours can help protect employees' sleep quality.