It means that she hopes you sleep well and have sweet dreams.
Similarly, what does it mean when a girl says Aww that's so sweet? Spending the night curled up in his arms may be the perfect ingredient for a loving relationship that's moving forward.
What does it mean when a guy tells a girl, you're sweet? Also it is a more familiar term also.
It means she's tired of the discussions and arguments.
The simplest answer is Good morning.
It is like saying hey or hi to a friend instead of hello.
It's a simple few words really, at a normal hour, when people in her past might not have done the same.
Erik on January 06, 2018: There this girl that responds very efusivelly to my texts: lots of emojis, like 6 in a row, some of them like the blured smiley, monkeys, hellooooo's, etc.