Aims: To explore whether awake and sleep bruxism interact in their associations with painful temporomandibular disorders (TMD) and whether the interaction is multiplicative or additive. Methods: In this case-control study, all participants (n = 705) were part of the multicenter Validation Project and were recruited as a convenience sample of community cases and controls and clinic cases. Logistic regression analyses were applied to test for the association between self-reported bruxism (sleep and/or awake) and the presence of painful TMD, and odds ratios (ORs) with 95% confidence intervals (95% CIs) were computed. Regression models included an interaction term to test for multiplicative interaction, and additive interaction was calculated as the relative excess risk due to interaction (RERI). Results: Based on logistic regression analyses adjusted for age and gender, the main effects for both awake (OR = 6.7; 95% CI: 3.4 to 12.9) and sleep (OR = 5.1; 95% CI: 3.1 to 8.3) bruxism were significant. While the multiplicative interaction (OR = 0.57; 95% CI: 0.24 to 1.4) was not significant, the results indicated a significant positive additive interaction (RERI = 8.6; 95% CI: 1.0 to 19.7) on the OR scale. Conclusion: This study has demonstrated that awake and sleep bruxism are associated with an increased presence of painful TMD, and that both types of bruxism are not independently associated, but interact additively. As such, the presence of each factor amplifies the effect of the other. Keywords: awake bruxism, interaction effect, pain, sleep bruxism, temporomandibular disorders