F.lux is a popular bit of software that adjusts the brightness of your screen based on the time of day. It will give your desktop screen warmer colors at later hours of the day, to make it easier on your eyes when you go to sleep.

The company recently pushed out a version of its software for devices running Apple iOS 9. The mobile software seemed to be doing well, until Apple told the makers of f.lux that the iOS iteration violated the Developer Program Agreement.

F.lux co-founder Michael Herf told Re/code that Apple representative Richard Chipman explained to him “we were not authorized to use the iOS SDK [standard developer kit] or Xcode [Apple’s developer tools] for the purpose we did, and we could not distribute f.lux as we did.”

Herf and his co-founder/wife Lorna worked at Google until 2008, after it acquired their photo storage startup Picasa in 2004. They say the desktop version of their app has been downloaded more than 15 million times, and that they were caught off guard by Apple putting the kibosh on f.lux for iOS.

To give some context, Apple began letting developers release stuff for iPads and iPhones outside of the App Store a few months ago. This meant that people could “sideload” apps instead of downloading software the conventional way from the App Store, provided they were willing to download Apple’s Xcode developer tools. Android users can sideload apps without needing special software, though in some cases they have to change a user setting.

“The last six months of ‘sideload’ press — which Apple didn’t try to stop — had convinced us that Apple would be receptive to an approach like this, but they seem to disagree,” Michael Herf said. “I asked him about open source used in a similar way, and he did not answer clearly, but he kept repeating the party line that we should make apps that could use Public APIs.

“Apple’s App Store Review process automatically rejects any app that calls ‘Private APIs,’ which f.lux does, because there is no Public API to do what f.lux does.”

So why might Apple want to kill something like f.lux for iOS? There are a couple possibilities: First, Apple has traditionally been opposed to any change to the look and feel of the iPhone beyond the capabilities that it specifically gives developers. That means, for instance, that apps can’t take over calling or change the default Web browser. Android, on the other hand, has given developers freer rein to do what they like.

A second idea is that Apple is thinking about the same stuff that f.lux is. CEO Tim Cook is talking a big game about the company’s efforts to expand into health technology. F.lux, citing a bevy of scientific research, says it is working on making screen light less harmful for you to look at. It makes sense that Apple would preemptively shut something like f.lux down if it’s planning to, say, unveil its own f.lux-like service in a future version of iOS; the platform giveth and the platform taketh away.

Herf says that it wouldn’t surprise him if Apple is working on something like this.

“Their new devices — 6s, 6s Plus — they’ve put in some color-matching features in the phones, but they don’t change their screen color,” he said. “When you poke around in their APIs, it’s not a big leap to say they’re thinking about the topic.”

Apple has not responded to multiple requests for comment.