My problem, ultimately, was that my SCM muscles were extremely overworked and felt like I was hiding two brautwursts in my neck - the muscles were so tight that they were pinching some kind of nerves and sending referred pain to my head and eyes. After changing back to a spring mattress the muscles softened and returned to normal and the pain subsided.

I think that since I am a side sleeper (and have desperately tried other positions with no luck), that my body was sinking in to the bed but my head was not and so my neck muscles were compensating to try to "hold" my head/neck all night long, causing them to become overworked and fatigued.

I suspect people who sleep on their back or stomach have different experiences; my wife sleeps on her back and found the Casper to be comfortable but ultimately she got tired of the 'soft' edge of the bed which was not easy to sit on without sliding off, so she shopped for a spring mattress - and once my pain subsided we were able to put the timeline together.

The way foam mattresses work is weight deformation. If your butt is heavier than your head, it sinks in more.

I’ve never seen any evidence to suggest this was good for spinal alignment. If you’re a side sleeper, it can also be bad for your shoulders if it doesn’t sink in enough.