Hi!

I believe your heart racing (tachycardia) is caused by it not getting correct blood volume flow from the vena cava, which major vein brings in deoxygenated blood in to get pumped to the lungs.
The key you mentioned is "an hour or so after lying down to sleep". The deep breathing likely works, but it could be mainly because you are now standing.

The vena cava running vertically is behind the liver's caudate lobe, and in front of something quite immovable and hard known as the spinal column with its vertebrae.

In some people, the caudate lobe at the very rear comes to a blunted rounded point, whereas in most this caudate lobe has a minor hemispherical "dent" in it, allowing it to pretty much snug around the vena cava (at that point it is about the thickness of a finger).
Only slightly explained here:
It is bounded, below, by the porta; on the right, by the fossa for the inferior vena cava; and, on the left, by the fossa for the ductus venosus.
http://en.wikipedia.org/wiki/Caudate_lobe_of_liver


A better explanation, complete with an exact anatomical picture, is in this book:

http://www.amazon.com/Secrets-Great-Health-Liver-Dwarves/dp/0968285309/ref=sr...

Here it is same idea but happening lower down, in 3rd trimester when baby needs room and it pinches off vena cava against the spine:



So, if the liver gets inflamed and increases volume, whether from toxins or disease, the caudate lobe in people who have the more pointed tip will pinch the vena cava against the spinal column, causing the heart to sense less blood and thus increase the heartrate to compensate!
This will of course happen mostly after a while horizontal, since the weight of the liver alone pushing down can do it.

Perhaps sleeping only SLIGHTLY reclined in a chair with sidearms so you can't fall out when asleep for even a night or two will tell you for sure if this is it.

Since you say you have Lyme, have you looked into a few key herbs that have been used for centuries to heal the liver, like milk thistle (silymarin), with its key component silibinin?
Don't worry about reading you may have an inflamed liver--it's the most healable organ in the entire body, moreso even than the skin. It has to be, since it does all the detoxing!