I've noticed this a lot more recently. After sitting or lying down for a while I get very lightheaded when I first stand up. My blood pressure is as good or better than ever. From what I found on the internet (of course it's never wrong) maybe my heart is getting stronger and my HR even slower. That could make sense with the increased exercise over the last several months. I also wonder how my dehydration plays in it.
I used not have the issue but I did really notice it while riding in the higher elevations of Colorado and leaning over to grab my water bottle. It really makes me wonder how bad it's going to be this summer.......
(btw, the scales were very kind to me this morning and only read 169. Just 4 more lbs to get to my all time low for over 20 years. Makes me wonder if I can get to 160???)
Here is one article that I found on a Sportsmedicine site:
If you exercise often and are in good shape you might occasionally experience dizziness and lightheadedness when you stand up quickly. This is generally nothing serious, and occurs due to a slow heart rate.
Cardiovascular exercise makes your heart stronger and a stronger heart has a larger stroke volume. That is, the amount of blood pumped out during each beat is greater, so the heart doesn't have to beat as often. A slow pulse rate is an indication of a strong, healthy heart. However, a slow heart rate can sometimes lead to dizziness when you change position.
When you stand up quickly gravity pulls blood from your brain towards your feet and blood doesn't return to the brain until the next heart beat. With a slow pulse, this takes a second or two and that is enough time to feel the lack of oxygen in the symptom of lightheadedness or dizziness. It is also related to something called postural hypotension. This results from a decrease in blood flow to the brain, due to a drop in blood pressure upon standing up.
As long as it occurs only occasionally, you don't really need to worry. If you have constant and severe dizziness with changing position you should see a doctor to rule out an underlying condition such as an irregular heartbeat.
Quick Tip: If you have a slow pulse (50 or less) and experience dizziness when standing up try getting up more slowly and see if that solves the problem.