The heaviness for me was due to lack of blood flow , lack of coordination followed if I continued the activity.

Ongoing heaviness , swelling & blue or mottled color changes in arms can be Venous TOS related..

In the past some had both NTOS & VTOS & Pec minor syndrome, or other combination of factors..
Like scalene issues, Extra C rib, large transverse process, large neck muscles or larger than normal bones..
A tricky process to figure out all of it.. if you can find out & work on the worst symptoms, the rest might follow.

The only thing that ever worked for me was stretching my pec minor daily and shoving my first rib down.

Well that and being so exhausted, I passed out. :D

Some people, including myself, have also slept fairly upright in recliners, with or without pillows under the arms.

HTH

Help for TOS pain while lying on back
 

Hi nickO and others, Sleeping on your back, or sides can make the pain in your arms much worse and prevent sleep. What has helped my symptoms, which are in both shoulders and arms, is sleeping on my back with both shoulders propped forward and a little bit up towards my head. I do this with a pillow that extends down under my shoulders, not just supporting my neck and head. If I fall off the pillow, my symptoms recur. If I prop them back up, they resolve. I also make sure my neck is slightly flexed by the pillow being tall enough or second narrow pillow under the very top of my head. All of this positioning makes the scalene neck muscles and the pectoralis minor muscle less taut. Hope it helps you sleep!

Physical therapy has been life saving for me. I spent two years trying to figure out what was wrong. Once I was diagnosed, my doctor started me with physical therapy. I've been going 2x per week since September 2019 and have noticed a big difference.

Hope your physical therapy has helped.