An optical circuit detects PPG signals reflected from skin tissue at one or more different wavelengths. A processing circuit integrated in the biosensor or in communication with the biosensor processes the PPG signals to obtain a period of vasodilation, a level of vasodilation and rate of change of the level of vasodilation. The processing circuit compares the level of vasodilation to a normal range and determines an arterial stiffness index using the comparison and the rate of change of the level of vasodilation.