Off the top of my head there are 2 major differences between e-ink and non e-ink displays that could potentially explain differences in comfort level for prolonged use:

- Non e-ink displays have a refresh rate, therefore continuous "activity" on the screen. People can have different sensitivities to this.

- Non e-ink displays tend to have higher contrast and/or brightness. This would normally be considered an advantage, but again perhaps not for prolonged use.

I don't need scientific papers to tell me I find e-ink more comfortable or not, it becomes very quickly apparent when I try to read a book on a tablet. Obviously you have a different perception, that does not invalidate mine.