Why does Apple not allow flux (http://stereopsis.com/flux/) for iOS?

Security being highest priority in iOS, each AppStore App is placed in a sandbox.  This means all the App's data and metadata are in a separate isolated box that has a very limited access (via interfaces) to outside files, system preferences and hardware, including display settings, network settings etc. Apps can, however freely access and organize their own data using local directories as shown in the figure above. Sandbox is good because it limits the damage that a compromised or malicious App can cause to the whole system. Jailbreaking unlocks some powerful capabilities in iOS for adventurous users to remove restrictions Apple has placed. It allows you to use your device more like an Android or a personal computer. However, I think the security risks and efforts of jailbreaking definitely outweigh its benefits. I haven’t jailbroken my own iPad nor am I likely to anytime soon.

Imagine a system where all software can have deeply significant influence on all other parts of the system.   We can imagine an app which sees through the camera, plays sound, accesses the finger print reader, and rewrites all the files on the drive.Alas, we already have computers like this, and they tend to become increasingly unstable over time.  Because software which can dick with the system ends up turning it into a slowly broken mess.   The accumulated cruft of well written software can do this.   With badly written or malevolent software it gets worse.So... iOS limits what applications can do. And these limitations keep the system stable.  There is a 50 foot high wall dividing third party apps from system level features.  So stuff which shows things on the screen and accepts user input can live on on the app side. But anything which changes the entire system has to live in on the OS side of the security wall.   Flux (or anything like Flux) is a OS type  feature.   

Because, based on my experience with the OS X version, f.lux is a severely broken piece of software; it causes "posterization" of anything using GPU-accelerated video rendering like videos in Safari or games so everything looks like a 1980s computer that only can show 16 colors at once.The good news is that the latest iOS betas have a similar feature built-in to the OS and Apple's version actually works.

Thanks for the A2A.Other answers explain why f.lux is not available on iOS, but you might be glad to know Apple is building in a similar functionality starting with iOS 9.3, which is in public beta right now.

The whole concept of "blue light" is just fundamentally stupid and I suspect the smart people at Apple would think the same and not implement such a feature just on that grounds. Ever notice how people texting at night have that eerie blue glow? Yes, it's because the color temperature of their internal lighting does not match the color temperature of their screen. Most phone screens are intentionally biased towards the blue side because it makes them seem brighter. Accurate screens (like the ones one iPhones and Samsung Galaxy S5 and latter) do not have such biases and don't look as blue. Most interior lighting are also made to be very warm, combine that with the very limited dynamic range of modern camera sensors, when they are adjusted to have the white balance of the lighting, it makes anything thats's true white look very bluish.A scene lit with daylight shows no problems.  Your typical "eerie blue blow" scenario. This is only because the camera is calibrated to the color of the light, which is way warmer than actual white. Adjusting the camera back to the same daylight standard shows that it's the lamp that's off, not the screen. This is a very well-understood phenomenal and the only thing it shows is that our camera sensors still need improvement. And many phone manufacturers still need to calibrate their screens in accordance with the sRGB standard. When looking at this scene with your meat eyes you won't see any problems. You'll notice that the lamp isn't perfectly white but not as orange as the picture suggests, and you DEFINATELY do not see like the second picture. This is because your eye has so much more dynamic range, the relative difference in color temperature is not as significant. During the day, computer screens look good—they're designed to look like the sun. But, at 9PM, 10PM, or 3AM, you probably shouldn't be looking at the sun. The problem with that is your interior lighting also doesn't change over the time of the day. If you have 5000k light bulbs it's 5000k 24 hours of the day so if your screen changed to a 2700k white point it will look super inaccurate and broken. The only way to implement this is to have RGB ambient light sensor built-in our devices to match the environment lighting. Software solutions that only change color based on time of the day will ultimately fail and only make colors look inaccurate.Also modern 8-bit screens have barely enough bit depth to work with, skewing that with color filters only makes it worse and significantly reduces display quality. .lux fixes this: it makes the color of your computer's display adapt to the time of day, warm at night and like sunlight during the day. At night there is no sun and there is no "correct" color temperature, if anything natural moon light is just as white at 6500k as daytime sunlight. So this argument is just retarded. Also during sunrise and sunset you don't see the sun as white, you see them as orange/red because your eye's ability to adjust white point is not fooled by time of the day like a camera would. If your screen changed colors you would also be able to see that, and you'd be able to see that the colors are off and stupid. It's even possible that you're staying up too late because of your computer. You could use f.lux because it makes you sleep better, or you could just use it just because it makes your computer look better. While there is some creditability to the idea that a warmer screen may help you get tired and sleepy and not mess with your biological clock as much. You know what's better? NOT TEXTING ENDLESSLY IN BED WHEN YOU'RE SUPPOSED TO BE SLEEPING. It is just questionable how much of an effect such a small light source like a phone screen can affect you. Even if you put it close to your face it's still doesn't take up much angular space. Also, what does this say about blue wallpapers (like actual physical papers on walls), curtains, or if your SO just like blue clothes? f.lux makes your computer screen look like the room you're in, all the time. When the sun sets, it makes your computer look like your indoor lights. In the morning, it makes things look like sunlight again. BULLSHIT it doesn't, it can't possibly know the lighting condition in my room. Again, the only possible way of doing this correctly is if you had an RGB ambient light sensor. AND this has to work at a hardware level, adjusting the hardware of the screen instead of messing with the software output based on some time table.It's a hack job of a solution to a minor problem, it wouldn't be in Apple's character to do such a poor job at this. It's like making the phone screen white as a flash, any hacker on XDA would be content with a software white screen. But Apple needs to build a custom driver chip into their phones to make the screen flash 3 times brighter than it can normally go, AND at the correct colour. Even according to their own research this is all useless and BULLSHIT, they talk about how brightness has a large effect on your biological clock. So guess what? TURN DOWN YOUR BRIGHTNESS? (seriously though, this solves like 95% of the problems people have with eye strain) Not only do you not have to run an app in the background and have it push ads to your phone, you also don't loose any image quality or performance. So if you like texting at night, go get yourself a Samsung Galaxy S7/S6/S5, its AMOLED screen is capable of a very very dim mode that's very comfortable even in pitch darkness. And you know what else is great about this? IT'S AUTOMATIC! Yeah! Your phone has a brightness sensor built-in and it automatically turns down your brightness if your room is dark, EVEN IF it's 12:00 PM! For desktop displays that don't go very dim, this may sound stupid but just put on some sunglasses, no seriously, it works with every application/fullscreen games. And also does not reduce image quality.