Personally I define something to not affect my sleep, when I got trouble staying awake after one or two chapters on a workday.
PRS-350 (no light eink): Like a real book, it mostly depends on the light setup of the room. With a warm, indirect and just enough to not have trouble reading dim light, I easily get sleepy and in trouble to stay awake long enough to poweroff device and lamp. I did fall asleep with device in hand often enough, even on a plane once.
Aura HD (light layer eink, one of the first gens): Had to stop myself reading often after looking at the clock or how many chapters I read. Just one more chapter, was a real problem with it. But I didn't had any trouble falling asleep after putting it away.
Note10 kobo reader (amoled, red text on black, "background light off mode"): Need more time than PRS-350 or real book, but I definitely got trouble keeping my eyes open after a while. Sometimes I switch it off and already fall asleep before I get the chance to put the phone out of my hand.
Note10 anime reader app/websites/etc: Even without videos or other animated/interactive media, I stay awake indefinitely. If I fall into that trap, it is common for me to stay awake to at least 2am, sometimes up to 5am.