I now theorize that I was wrong. My current theory is that I'd been watching videos, which are obviously not all emitting pure white or blue light all the time. Much of the time, indeed, the colors on the screen can be quite dark.
The same is not true if you're using the iPad as a reading device. Then, it's straight-up black text on a white background. Then, you stay up until 2 am reading. Then, you can't fall asleep before 2 am for the next two weeks; and then, the trend shows no sign of stopping.