Introduction and objective:
Obstructive sleep apnoea (OSA) often co-occurs with multiple comorbidities. The aim of the study is to examine whether stress mediates the relationship between comorbidity burden and psychological functioning in OSA patients, and whether optimism moderates these relationships.

Material and methods:
Sixty OSA patients aged 27–81 (45 males, 15 females) completed questionnaires assessing stress (PSS-10), optimism (LOT-R), life satisfaction (SWLS), and mental health (GHQ-28). Patients were categorised into groups with and without comorbid conditions; the number of comorbidities was treated as a continuous variable. The relationships between variables, as well as mediation and moderation effects, were assessed using Partial Least Squares Path Modelling.

Results:
Patients with comorbidities reported higher stress than those without (mean = 16.38 vs. 12.077; p = 0.039). Comorbidity count correlated with stress levels (β = 0.358; p < 0.01). Stress predicted life satisfaction (β = -0.503; p < 0.001) and correlated with mental health disorder symptoms (β = 0.564; p < 0.001). Stress fully mediated the relationship between comorbidity burden and both life satisfaction (β = -0.180; p < 0.01) and mental health problems (β = 0.202; p < 0.01). Optimism moderated the stress-life satisfaction relationship (β = 0.206; p < 0.05).

Conclusions:
Stress mediates the relationship between comorbidity burden and psychological functioning in OSA patients. Hence, stress screening and management should be incorporated into OSA care, particularly for patients with multiple comorbidities. The results obtained support the holistic approach to OSA management addressing the psychological burden of multimorbidity.