I know this, too. But I also know types of eye strain that kick in immediately, and/or over time. For example, the pixel-induced eye strain we don't know the exact causes of yet kicks in immediately. PWM and other flicker may cause immediate symptoms (for example eye strain or feeling sick) or slightly delayed symptoms (like headaches later on). And looking at a flicker-free white LED display or using LED room lighting can have delayed effects, too (eye strain, burning eyes, headaches).
But what they have in common for me is the next time I wake up, I wake up with eye strain and burning eyes, stronger than the day before when I was exposed to the flicker or light. It is difficult to get up then, and I also need more sleep than usual, making it even more difficult to get up in time.
This is debilitating, because even when turning off the display or lamp rather quickly, the symptoms not only persist for a few hours after exposure, but they make it into the next day as well.