A non-invasive blood pressure variability measuring system, comprising: a cuff pneumatically connected to a pump arranged to inflate the cuff, the cuff to be wrapped around a limb of a subject; a pressure sensor associated with the cuff arranged to measure a cuff pressure; a photoplethysmogram sensor to be attached to a fingertip in the same limb of the subject and to be placed to be distal to the cuff, the photoplethysmogram sensor arranged to monitor blood flow and to record a pulse plethysmograph signal; and a control unit connected to the pressure sensor and the photoplethysmogram sensor, the control unit configured to: inflate the cuff from a low pressure below the diastolic pressure, or to deflate the cuff from a higher pressure above the systolic pressure in steps and hold at a constant level at each pressure step for a duration more than one respiratory cycle; simultaneously record the cuff pressure and the plethysmograph signal; and derive an empirical relationship between the cuff pressure and an amplitude measure of the plethysmograph signal by obtaining a distribution of the PPG amplitude measure at each cuff pressure step and a set of PPG amplitude measures at different cuff pressure steps, and using this distribution to estimate a short term variation in systolic and diastolic blood pressures at a frequency corresponding to that of a respiratory cycle; wherein the empirical relationship between the cuff pressure and the amplitude measure of the pulse plethysmograph signal is determined using a parametric curve fit in order to yield a mean value curve and variation curves providing a variation in pressure for each value of the pulse plethysmograph signal, and wherein the mean value curve and the variation curves provide the distribution of the systolic and diastolic pressures and an average pulse pressure. a cuff pneumatically connected to a pump arranged to inflate the cuff, the cuff to be wrapped around a limb of a subject; a pressure sensor associated with the cuff arranged to measure a cuff pressure; a photoplethysmogram sensor to be attached to a fingertip in the same limb of the subject and to be placed to be distal to the cuff, the photoplethysmogram sensor arranged to monitor blood flow and to record a pulse plethysmograph signal; and a control unit connected to the pressure sensor and the photoplethysmogram sensor, the control unit configured to: inflate the cuff from a low pressure below the diastolic pressure, or to deflate the cuff from a higher pressure above the systolic pressure in steps and hold at a constant level at each pressure step for a duration more than one respiratory cycle; simultaneously record the cuff pressure and the plethysmograph signal; and derive an empirical relationship between the cuff pressure and an amplitude measure of the plethysmograph signal by obtaining a distribution of the PPG amplitude measure at each cuff pressure step and a set of PPG amplitude measures at different cuff pressure steps, and using this distribution to estimate a short term variation in systolic and diastolic blood pressures at a frequency corresponding to that of a respiratory cycle; wherein the empirical relationship between the cuff pressure and the amplitude measure of the pulse plethysmograph signal is determined using a parametric curve fit in order to yield a mean value curve and variation curves providing a variation in pressure for each value of the pulse plethysmograph signal, and wherein the mean value curve and the variation curves provide the distribution of the systolic and diastolic pressures and an average pulse pressure. inflate the cuff from a low pressure below the diastolic pressure, or to deflate the cuff from a higher pressure above the systolic pressure in steps and hold at a constant level at each pressure step for a duration more than one respiratory cycle; simultaneously record the cuff pressure and the plethysmograph signal; and derive an empirical relationship between the cuff pressure and an amplitude measure of the plethysmograph signal by obtaining a distribution of the PPG amplitude measure at each cuff pressure step and a set of PPG amplitude measures at different cuff pressure steps, and using this distribution to estimate a short term variation in systolic and diastolic blood pressures at a frequency corresponding to that of a respiratory cycle; wherein the empirical relationship between the cuff pressure and the amplitude measure of the pulse plethysmograph signal is determined using a parametric curve fit in order to yield a mean value curve and variation curves providing a variation in pressure for each value of the pulse plethysmograph signal, and wherein the mean value curve and the variation curves provide the distribution of the systolic and diastolic pressures and an average pulse pressure.