I used to get this a LOT when I was younger, though not recently.

It sounds like your heart is entirely fine (from the tests etc), and this is some form of anxiety.

Do you always take your blood pressure when you wake up? How often a day do you take your blood pressure? (This sort of behavior demonstrates anxiety motivated behavior).

I have no idea why it used to happen when I was asleep but if you worry about it and fear it, that is going to make it worst.

The only advice I can give is to try and stabilize your sleep and reduce the stress in your life.