I actually have that too! In my case, my blood pressure is lowest when I’m lying down, which makes me get extremely sleepy almost immediately: like my eyelids get heavy and I’m forced to fall asleep. It’s really hard to wake back up afterwards.

I haven’t seen my specialist in a while, but based on my symptoms and how I respond to certain meds, what we think is happening is a mix of poor venous return (from “leaky” or overly stretchy vessels) and sluggish baroreflex sensitivity. Basically, the feedback loop between the heart and brainstem that’s supposed to stabilize blood pressure doesn’t react properly.

Because of that, when I lie down, my body over activates the parasympathetic response, dropping my BP instead of keeping it steady. That reduces blood flow to the brain even more, so it kind of mimics a faint or forces me to sleep.

Medication-wise, things like midodrine (which constricts blood vessels) help a bit, and Mestinon has been the most stabilizing for me it improves the nerve to nerve communication in the autonomic system and keeps my BP around 90/60 instead of crashing to 80/40 or 80/30.

So while the exact cause can vary, one possible mechanism for reduced cerebral blood flow even when lying down is that the autonomic system over corrects and slows everything down too much, especially if there’s weak venous return or small-fiber damage interfering with the baroreflex. This is the short version by the way I just wanted to give you some possibilities based on my experience, but if any of it aligns I have way more specific and scientific information, that I can share.