What Causes Light Headedness On Waking Up In The Morning?

I have been waking up lightheaded in the morning, and anytime I take a nap. My psychiatrist said I was dehydrated and that may be causing it. I m now drinking two liters of water a day and I have had no change. It only happens upon waking up, and all it takes to start the lightheadedness is to lift my head off of the pillow. It usually only last a few minutes, but have had it last over an hour. Cat scan came back normal, as has my blood work, and my EKG. I do take Wellbutrin XL, and Klonopin. I take them for bipolar and panic disorder. I don t think it s related to my meds because I was on the same combination before for six months without being lightheaded. After it subsides, I have no problem doing any standing from a sitting position, no matter how fast I stand up, or from any type of movement. My blood pressure changes very little during the lightheadedness. What could be causing this to happen everytime I wake up?