Anyone notice correlation between PEM and holding one's head up?

Something I've noticed lately is, seemingly, a correlation between me holding my neck and head being erect and unsupported, and PEM occurring.

The best example I have is noticing that I can work on my laptop for hours lying down, but surely less than 30 minutes for sure while sitting up at my desk. It's similar with watching TV: if I watch on my laptop lying down it's no problem, but watching a 45-minute episode of something sitting up on the couch can cause symptoms to flare up.

Has anyone else noticed anything like this for themselves?

(I am subclinical for POTS, if that helps. I don't notice much of a heart rate change between the two positions.)