[lmm]

I don't know about the iOS walled garden, but more generally there are tools to adjust screen colour temperature to something appropriate to the time of day.

[socket0]

There was: http://9to5mac.com/2015/11/12/flux-iphone-sideloading-shut-d...

[cableshaft]

Even if I set the iPad to its lowest brightness, though, it's still about 10x brighter than the lowest I can set the backlight in a Kindle Paperwhite.

[stevewillows]

IOS 9.3.4b also has automatic temp control built in. I was sideloading f.lux -- but this works just as well.