What Causes Headache And High BP In The Morning?

I often awake with a headache at around 4am I get up and it gets better. Two days ago I started monitoring my BP starting at around 8am and at that time of the morning is has been 206/120 and 190/120 then went down over a period of 3 to 4 hours to 150/70 and 160/96. What could be causing this? .