[Effect of body position changes and circadian rhythm on cerebral blood flow velocity]

To study the effects of -6 degrees head down tilt (HDT), upright posture and 24 h circadian rhythm on cerebral blood flow (CBF) velocity in humans, the CBF velocity was measured by use of transcranial Doppler technique in the middle cerebral artery (MCA) of 12 healthy male subjects. The results showed that the mean CBF velocity increased significantly during -6 degrees HDT and decreased during upright posture, and CBF velocity was higher in day time than at night.