Experimental data (starting from 1995) have shown that the volume of blood flow in human faces tends to oscillate - rising and falling once every ten seconds or so.
The reasons are unknown. There are currently two theories :
1) That it stems from the blood pressure regulation mechanism
2) That it's caused by 'vasomotion' (contraction and expansion of blood vessels)
See: Phase Velocity of Facial Blood Volume Oscillation at a Frequency of 0.1 Hz Frontiers in Physiology, 28 Jan. 2021
Note : The highly complex mechanism which controls blood pressure is itself poorly understood. See
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