Why is this the answer?

The answer is sunrise because this clever riddle uses imaginative words to describe the short, special time when the sun first comes up in the morning. Think about the first line: "Sleeping during the day, I hide away." This clue is actually talking about the *event* of the sunrise. When the sun is high in the sky in the middle of the day, the time for the sun to *rise* is finished. The word "sunrise" means the moment the sun appears, and that moment is not happening in the afternoon. So, the moment of sunrise is "hidden" or "sleeping" while it is daytime. It is simply waiting for the night to end. Next, the riddle says: "Watchful through the night, I open at dawn's light." During the dark night, the sun itself is still shining, but we cannot see it because the Earth has turned away. The phrase "watchful through the night" means that the time for the sun to appear is getting closer and closer. The sunrise is waiting patiently. When dawn's light finally begins to color the sky, that is the signal for the sunrise to happen, to "open" and show itself over the horizon. The most important clue is the next line: "But only for the briefest time, do I shine." The actual moment when the very top of the sun peeks over the horizon and then slowly climbs up is very quick. It is a brief, short moment of shining that is called the sunrise. If you try to watch a sunrise, you know the most exciting part is only a few minutes long. And the last part, "And then I hide away. And sleep through the day," goes back to the idea that the *event* of the sun rising is now over. Once the sun is fully up and bright, we do not call it the sunrise anymore, we just call it "daytime." The quick, beautiful moment of the "sunrise" has hidden itself and is resting until the next morning comes. This riddle is a lovely way to think of the daily cycle of the sun as a quick, magical moment that happens when the world wakes up.