Sleep disorders continue to be the most unrecognized modifiable risk factor for stroke. The relationship between sleep disorders and vascular risk factors and stroke has been well-documented but not fully understood. The investigators hypothesize hemodynamics impairment to be its potential mechanism. It has been reported that sleep-related breathing disorder, a type of sleep disorders, contributed as a risk factor for stroke through hemodynamic and hematologic changes. The purpose of this study is to determine the relationship between sleep disorders and cerebral hemodynamics. The dynamic cerebral auto-regulation (dCA) and Transcranial Doppler (TCD) will be used to evaluate cerebral hemodynamics.