I find that my blood pressure varies a great deal. It could be 160/90 and then I can lay down and it goes down to 130/76. Last night it went down to 117/67 while I was laying down and when I sit up or stand up it goes up. It went up to 130/84 just by sitting up after the lower reading and in the morning it is the highest. Usually around the 160/90 rage. Any explaination for that?