Inclusion Criteria:  hypercapnic respiratory failure (PaCO2 > 6kPa) low usage of home mechanical ventilation (NIV < 4hours/night). body mass index (BMI) >18.5kg/m2 all genders age 18-90years.  Exclusion Criteria:  no hypercapnic respiratory failure (PaCO2<6kPa) no sleep-disordered breathing (AHI<5/hour) exclusively obstructive sleep apnoea isolated Rapid-Eye-Movement (REM) sleep associated OSA cachexia (BMI <18.5 kg/m2) severe pulmonary hypertension valvular heart disease heart failure (New York Heart Association, NYHA III-IV) myocardial infarction and significant cardiac arrhythmias uncontrolled hypertension active psychiatric disease co-existing non-respiratory sleep disorder.