I have a theory about this as I suffer from dreadful sleep issues and can be too wired to sleep and have horrid adrenaline surges. If I stand sit and lie down the difference in my BP is incredible. Standing is often 73/58, sitting 90/60 and lying 115/70. I have ME as well as dysautonomia. I think if I am upright for too many hours without getting flat, even for just 10 minutes, or let my fluid intake fall, the only way blood is getting round is by adrenaline. I think it is keeping me alive and has done for 27 years since diagnosis. However at bedtime this is all running through my system and causes surges and even panic attacks. If I do something really silly like have a hot bath before bed this will make things so much worse, I will literally be awake for hours in a wired state. I think the only success I have had is with very calm evenings and early nights, and nothing that triggers adrenaline so no talking on the phone or eating late meals and a host of other things that increase this likelihood which can be terrifying.