As someone who is a visual designer (and I often create or modify icons at work for apps and the web), here are my thoughts:
- The lamp isn't clear enough. First of all, lamps look different all over the world. Second of all, it makes it seem like the iPad now has a lamp function, and on the iPhone it competes with the meaning and clarity of the flashlight icon.
- The eye icon with the moon is better, but it's not exactly clear what it does or modifies. It reminds me of some sort of private browsing mode.
- The latest icon plays on something that has been used for a long time: The brightness setting indicator on computer and TV displays, or in the Mac OS software when pressing the brightness buttons. It's more clear that it has something to do with modifying the brightness at night. In this case it might not be truly changing the brightness, but the perceived brightness of the colors due to the human eye and how it perceives warm tones as being less harsh and non-interfering with our circadian rhythm. For that reason I think it is the most successful of the three.
Can you tell that I've sat through plenty of critiques in my day? Hah.
My biggest problem with the whole thing is that the toggle for this setting isn't at the top with other setting toggles, but instead at the bottom with the quick actions. They don't belong together. Furthermore, Apple should let us customize which setting toggles and quick actions we want in our Control Center.