Huh? I never read studies about it, but no hypothesis?

I frequently stumbled upon the assumption that a LCD screen is pulsed and flickers and that makes all the difference as E-Ink is more steady. (Artificial lightsources can also flicker, but with reflection it evens out)

In (old?) theory too fast for the eyes to notice, but I surely notice a difference.

I expect most eInk users use a frontlight, many/most of which are PWM-controlled.