The investigators hypothesize that obstructive sleep apnea (OSA) contributes to impaired glucose homeostasis and associated vasculopathy, and nCPAP treatment of OSA should improve glycemic control and vascular function in OSA patients with type II diabetes mellitus. This study aims to investigate the therapeutic effects of nCPAP on glycemic control and vascular function in patients with OSA and type II diabetes mellitus.

Obstructive sleep apnoea (OSA) has been reported to be common (17%) in patients with diabetes mellitus (DM). Both OSA and DM are highly associated with cardiovascular morbidity and mortality. There is growing evidence that OSA may trigger or worsen pre-existing adverse metabolic profile indicative of cardiovascular risk. Treatment of OSA with nasal Continuous Positive Airway Pressure (nCPAP) has been shown to reduce blood pressure and hence to reduce the risk of atherogenesis. In patients with DM, the therapeutic effect of nCPAP is still not known, it would be important to delineate any independent effect of OSA on DM and the therapeutic effect of nCPAP on glycemic control to reduce the long term risk of macrovascular and microvascular complications.

Inclusion Criteria:

1. Patients with type II DM on a stable medication regimen (on diet / oral hypoglycaemic agents / insulin injections)
2. Age 25 - 70 years
3. HbA1C > 7%
4. AHI >= 15
5. Able to give written informed consent

Exclusion Criteria:

1. Patients with severe co-existing illness or poor functional performance
2. Patients with peripheral vascular diseases, vasculitis / Raynaud's syndrome or thrombocytopenia
3. Sleep disorders other than OSA
4. Patients who refuse nCPAP treatment for OSA
5. Excessive sleepiness causing potential harm (e.g. driver)
6. HbA1C >=7%
7. Habitual drinker (defined as more than 3 times a week)
8. Pregnant or lactating women