Sleep affects brain function and may contribute to vascular cerebral pathology through a diversity of direct and indirect mechanisms. Circadian rhythm investigation shows increased incidence of stroke between 6 AM and 12 noon. Risk factors for stroke such as high blood pressure, ischemic heart disease, and diabetes are modified by sleep and sleep apnea. Epidemiological studies have shown a dose-response relationship between the severity of sleep apnea and the odds ratio for development of systemic hypertension. There is now evidence of a causal relationship between sleep apnea and stroke. Following stroke, both in the acute and chronic stages, patients have a high prevalence of sleep apnea that reduces the potential for rehabilitation, further increases the risk of secondary stroke, and heightens mortality. Successful correction of sleep apnea with noninvasive positive airway pressure ventilation lowers mean blood pressure, and indirectly lowers the risk of stroke. Unfortunately, patients with stroke tolerate positive noninvasive ventilation poorly, and other means of correcting sleep apnea need to be investigated.