The hypothesis of this study is that either obstructive sleep apnea (OSA) or cigarette smoking (CS) exposure would produce oxidative stress and inflammation leading to endothelial injury, and the combined exposure would be additive or synergistic.

Inclusion Criteria:

• No acute illness
• No hospitalization in past 4 weeks
• Absence of medical comorbidity such as hypertension, ischemic heart disease, diabetes and hyperlipidemia, renal impairment, vasculitis or Raynaud's syndrome, peripheral vascular disease especially Buerger's disease, aortic disease (aortic aneurysm or dilatation), cancer, thrombocytopenia, coagulation disorder or other chronic medical illness

Exclusion Criteria:

• Mentally incapacitated

Consecutive subjects who undergo polysomnograms (PSG) in the Sleep Disorder Centre, Department of Medicine, Queen Mary Hospital, and who fulfill the study criteria will be recruited.