For years, I avoided e-ink readers (kindle, nook, etc.) because I hated looking at them when I really wanted to read - at night, usually before bedtime. When they didn't have a backlight, they required me to get up to turn on a light (and eventually turn one off). Similarly, I never liked using an ipad/android tablet for books because they usually felt too bright or bulky. This pain finally seemed to go away when the kindle and nook both introduced backlit e-ink readers.
So, I caved and impulsively bought a nook touch with glowlight. Using it during the daytime is great just like any other eink device, but something still didn't feel right when reading at night - my eyes just hurt reading it during the night.
Color temperature and your sleep
The problem I had with the nook glowlight (and the kindle paperwhite) is that from what I've seen, the color temperature felt too much light daylight. While this is great for daytime indoor use, it's not the type of light you should be staring at during the night. The main reason boils down to blue light delaying your sleep. I'm not going to dive too far into the science behind this, because there's tons of scientific papers referenced at the website for f.lux - a program I highly recommend if you want a similar lighting effect on your computer.
The solution - kapton tape
I wanted to change the color of the lighting of the nook at night, but I didn't want to just slap a dorky orange film over the screen. I wanted to change the light color at the source, without going to the extreme of changing the LED's. After playing with colored paper and various films, I remembered a type of tape I used all the time in electronics work - kapton tape! Kapton tape is a good electrical insulator, but more importantly it's translucent with a nice amber hue - perfect to mask blue light.
A small reel of kapton tape can usually be found on amazon or ebay for a few bucks.
Less talk, let's hack!
To change the color temperature, you need to fully disassemble your nook:
You'll need to follow the ifixit guide all the way through step 14, where you separate the plastic infrared touch bezel from the e-ink screen. I advise being really careful in steps 13 and 14, being careful not to ruin the black dual-sided tape that runs along the border. If it feels hard to do, I advise switching tools to use a small needle, exacto knife, or blade edge. Basically, you don't want to apply too much pressure away from the screen, because you don't want to separate the thin plastic cover that's glued to the eink screen. Also, try not to start peeling at the upper right corner, that's where the tape loop starts and ends.
Once you finally separate the screen from the bezel, you'll see a small strip of black tape across the tape with numbers. Carefully peel this tape off and you'll see the LEDs.
Now, grab that kapton tape. Cut it into small squares the width of each LED, and make it long enough to go into the side (where the led will be shining most of it's light) and have enough area to stick on top. That is, you're going to have small squares where each square is in an L-shaped formation.
Finally, start trying to stick the kapton tape. The key here is to stick the tape inbetween where the led actually outputs light and the plastic 'diffraction grating' cover. This is a really thin space, so it may take a few tries.
It's worth testing the lights before fully reassembling your nook - notably because it's hard to tell if you applied the tape far down in the light channel to cover the entire LED. You can connect your nook back up such that you have at least the display ribbon cables & battery hooked up, then after booting up hold down the main button twice to see what the light output will be.
After Everything looks good, I recommend putting a layer of kapton tape the length of the screen over the area between the diffraction grating and the LEDs. This is to make sure the individual tape pieces don't jump out of the gap between the cover and LEDs over time.
I honestly have no clue about whether this hack will work on the kindle paperwhite, I briefly played with one in a store and felt like it had the same lighting issue. I'd love to see someone try the same hack on a kindle though. ( update 08/07/13 - Nathan from the-ebook-reader.com pointed out that the new kindle paperwhite & kobos have 'yellower' backlights, hopefully the Nook folks follow suit.)
I ended up getting rid of the nook after a month, while it's a very hackable device, I got a weary at the dozens of bad reviews concerning it's poorly glued touch cover, which tends to separate over time and leave what looks like a hole in the middle of the screen. I was already having a similar issue before the hack, as seen in the upper right of the lit photos.
Finally, if you're still curious about how humans and other lifeforms react and depend on light, check out this Peter Warshall's podcast talk "Enchanted by the Sun: The CoEvolution of Light, Life, and Color on Earth"
I Hack, Tinker, and Simplify without saying "Simple".