Sudden cerebral events such as strokes, transient ischemic attacks and syncope are most common in the morning. There are two critical processes which are important in the pathophysiology and the avoidance of cerebral events: Cerebral autoregulation and cerebrovascular reactivity to carbon dioxide. Cerebral autoregulation refers to the ability of cerebral blood flow (CBF) to remain relatively constant within a range of blood pressures. If cerebral autoregulation fails the brain is at risk of ischemic damage at low blood pressures and of hemorrhage at high blood pressure. Cerebrovascular reactivity to carbon dioxide is a vital homeostatic function that helps regulate central pH and breathing; its impairment has also been linked to an increased risk of ischemic stroke and syncope. Although the central signaling pathways are not yet understood, it is known that CBF, autoregulation and cerebrovascular reactivity are impaired in the early morning when compared to any other time of day. We have shown recently that such reduction in cerebrovascular function, partially mediated via changed in sympathetic nerve activity, are reflected in marked reductions in orthostatic tolerance and related syncope at this time of day. We have also separated the effects of prior nocturnal sleep and time of day itself on haemodynamic responses to posture change. Moreover, such early morning reduction in cerebral autoregulation and reactivity may facilitate the onset of cerebrovascular accidents; this may be of particular relevance to at-risk groups, especially upon resuming the upright position.