Well, changes in blood pressure during sleep, particularly downward changes, creates a need by the body to adequately prepare you to rise from a lying position. Understand that if this did not occur, you would faint in almost every instance, or at the least feel light-headed and expeirence mild confusion. The longer you are in the suppine, or lying, position the more the body will compensate in order to make certain you can rise to your feet and perform safely.

If your blood pressure is very low when you're lying down, even when awake and you promptly stand to your feet, your body responds by increasing heart rate and blood pressure to the extent necessary.

This does not indicate the presence of a problem. What typically accelerates the heart beyond its normal response is the fear that something is not right, which naturally propels the heart as a response to epinephrine (adrenaline) in addition to any other increase already underway.

You'll be fine,

Best regards and Good Health