Introduction People with co-morbid insomnia and sleep apnoea (COMISA) have worse symptoms and require a tailored therapy approach for their sleep disorders. The relationship between the respiratory arousal threshold, a key OSA non-anatomical contributor and insomnia symptoms is unclear. This study investigated the relationship between insomnia symptoms using the insomnia severity index (ISI) and the respiratory arousal threshold in people with OSA. Methods 46 people with OSA (11 female, age=50±15years, BMI=29±6kgm-2, AHI=33±20events/h) participated in a physiology sleep study to predict response to oral appliance therapy and completed the ISI. Standard polysomnography, an epiglottic airway pressure sensor, bipolar intramuscular electrodes inserted into the genioglossus and a nasal mask with a pneumotachograph to quantify airflow were used. The respiratory arousal threshold was measured as the nadir epiglottic pressure during a respiratory event prior to a cortical arousal. Following this, participants trialled mandibular advancement therapy. Results There was no relationship between ISI (average=12±6) and the arousal threshold (-24±10cmH₂O) in participants with OSA (r=0.1, p=0.42, n=46). However, in the 59% of people with COMISA (ISI ≥11), there was a positive relationship between ISI and arousal threshold, with increasing insomnia severity associated with lower arousal threshold (r=0.5, p=0.017, n=27). People with a BMI<30kgm-2 (n=27), also had a positive linear relationship between ISI and improvement in oral appliance therapy (r=0.49, p<0.01, n=27). Conclusion Greater insomnia symptoms in people with COMISA is associated with a lower respiratory arousal threshold. In non-obese people with OSA, higher ISI values are related to poor response to oral appliance therapy.