Circadian and Diurnal Regulation of Cerebral Blood Flow

Abstract

Circadian and diurnal variation in cerebral blood flow directly contributes to the diurnal variation in the risk of stroke, either through factors that trigger stroke or due to impaired compensatory mechanisms. Cerebral blood flow results from the integration of systemic hemodynamics, including heart rate, cardiac output, and blood pressure, with cerebrovascular regulatory mechanisms, including cerebrovascular reactivity, autoregulation, and neurovascular coupling. We review the evidence for the circadian and diurnal variation in each of these mechanisms and their integration, from the detailed evidence for mechanisms underlying the nocturnal nadir and morning surge in blood pressure to identifying limited available evidence for circadian and diurnal variation in cerebrovascular compensatory mechanisms. We, thus, identify key systemic hemodynamic factors related to the diurnal variation in the risk of stroke but particularly identify the need for further research focused on cerebrovascular regulatory mechanisms.

Keywords: blood pressure; cardiovascular diseases; cerebrovascular circulation; heart disease risk factors; risk factors.

Figure 1.

Haemodynamic diurnal variation. Diurnal variation at rest in (A) mean arterial pressure (blood pressure), (B) heart rate, and (C) middle cerebral artery velocity. (C) is adapted from Conroy et al with permission. Figure credit: Ben Smith.

Figure 2.

Diurnal variation in systemic and cerebral mechanisms determining cerebral perfusion. The direction of change in each index is shown for night, morning, and afternoon, with the magnitude of change indicated by the number of arrows. Figure credit: Ben Smith. BP indicates blood pressure; FMD, flow-mediated dilatation; fMRI, functional magnetic resonance imaging; MCA, middle cerebral artery; and TCD, transcranial ultrasound.