The high frequency of unplanned hospital visits of patients with chronic hypercapnic respiratory failure (e.g., chronic obstructive pulmonary disease, obesity-related hypoventilation) constitutes a major public health problem. Most patients admitted for acute exacerbations (AHRF) have additional comorbidities, especially sleep disorders. Often untreated, sleep disorders contribute to multiple readmissions (≈70% at one year) and increase readmission costs. The investigators will: 1) identify these patients early during unplanned hospital admissions and perform sleep studies using EEG and oximetry before hospital discharge and two months after to compare sleep abnormalities in the two moments; 2) investigate the association between sleep abnormalities in the two sleep studies with clinical outcomes (1-year readmission and death); 3) investigate the acute effects of high-flow nasal cannula (HFNC) to treat sleep abnormalities as a simplified alternative. The investigators anticipate sleep abnormalities during the hospital stay and two months after discharge will be associated with poor clinical outcomes (readmission, death) and HFNC to acutely reduce sleep abnormalities.

The investigators will perform sleep studies using nocturnal EEG measurements and oximetry 1) while the participants are still in the hospital but have reached relative clinical stability (≥48 h after admission, not in ICU and pHv ≥7.36) and 2) two months after hospital discharge at the participant's home. Sleep studies will be performed from 5:00 pm to 8 am. EEG measurements will be performed using a portable monitor (Prodigy Sleep System, Cerebra, Winnipeg, Canada). Sleep assessment will be performed offline. The ORP will be continuously quantified and ORP-derived indices will be calculated. Nocturnal oximetry will be used for detecting sleep desaturation episodes. Comorbidities (e.g., obesity, diagnosed lung or heart disease, Charlson comorbidity index) and exacerbations/hospitalization in the previous year will be identified.

In 21 participants, the investigators will perform one additional sleep study with the participants breathing with high-flow nasal cannula therapy (HFNC) to investigate the acute impact (i.e., during one night) of HFNC on reducing sleep abnormalities. HFNC will be set from 25 - 60 L/min based on participant's tolerance and the inspired fraction of oxygen will be titrated to maintain patient SpO2 similar to the levels during standard oxygen therapy.

Inclusion Criteria:

• Adult patients requiring an unplanned hospitalization or emergency room visit due to an acute hypercapnic respiratory failure exacerbation (AHRF), not on home noninvasive or invasive ventilation, neuromuscular disease (e.g., spinal cord injury) or central nervous system disorders (e.g., cerebral vascular accident, Alzheimer), absence of tracheotomy, and non-morbidly obese.

Exclusion Criteria:

• Patients with AHRF due to drug overdose
• Ventilatory support (noninvasive or invasive ventilation) is needed before or during the sleep studies or if an acute deterioration (e.g., cerebral vascular accident, cardiac arrest) happens before or during the sleep studies.

Patients with acute hypercapnic respiratory failure (pH <7.35 and PaCO2 > 45 mmHg at admission) admitted for an unplanned hospitalization or emergency room visit.