I read before bed but am unsure if I should be using a no back light Kindle or if the Kindle app on my iPhone is ok? Anyone have experience with sleeping quality after reading using either of these?

I think it depends on if you feel it's keeping you up or causing eye strain. I find a backlit Kindle has a decent amount of brightness and is easy on the eyes.

Using an iPhone I'd be more worried about distractions than anything else

I read every single night on my phone with a black screen and white letters. It helps me to fall asleep every single night. I usually only get about halfway through a chapter before I pass out with my phone in my hand. Really It's just up to how it affects you.

I listen to audiobooks while falling asleep. Best of both worlds. You're not "wasting time" waiting to fall asleep, and you're reading. And if you fall asleep while reading, all the better! Just jump back to what you last remember tomorrow.

I'll say that this works best with books I've already read.

Another vote for a backlit e-reader. Much easier on my eyes in a dark room than the dark mode on my phone.

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Since your Kindle doesn't have a backlight (and especially the warm light), any difference really depends on the other lighting you are using. A dim, warm house lamp is probably better for sleep hygiene than the phone even with it changing its color temperature. If you have no problem falling asleep with either one then I don't think it matters much.

The truth is that using a backlit screen before bed will affect everyone differently. Try both and see which works better for you.

I have been reading on my phone before falling asleep for many years now. I use an app with amoled black background and yellowish peach font colour. I haven't felt any eye strain or difficulty with sleep.

For me, yes. I notice that whenever I'm reading on my phone (night mode, blue light filter, lowest light setting), I will stay awake longer, my eyes strain, it takes me longer to fall asleep. When using my kobo (not backlit, but still lit - it has those leds all around the screen that light it up from above!!), it's like I was reading a paper book, except I can read in a dark room without any strain to my eyes whatsoever, and I can take on any position without having to adjust the light source!!! An absolute game-changer in terms of comfort. In fact, I think my eye strain is even lower with the e-ink reader compared to regular paper indoors, because I have a better light source no matter where I sit in my room.