OBJECTIVE: To examine the effects on firefighters\' nocturnal cardiac autonomic activity and sleep quality of one on-call night without intervention and one on-call night with intervention. METHODS: Thirteen firefighters completed three experimental nights: a control night (CON), an on-call night without intervention (0-INTER), an on-call night with one simulated intervention (1-INTER). Sleep parameters were determined from nocturnal heart rate variability (HRV), objective and subjective sleep quality. RESULTS: Derived parasympathetic HRV indices were higher in CON compared with 0-INTER and 1-INTER (P < 0.05). Subjective sleep quality and total sleep time were decreased in 1-INTER compared with CON and 0-INTER (P < 0.01). DISCUSSION: These results revealed that for firefighters, being on-call during the night with and without interventions disturbs cardiac autonomic activity. Objective and subjective sleep quality were disrupted when on-call nights were interrupted by simulated firefighting interventions.