Waking up to a flirty text from your crush first thing in the morning is exactly the opposite of getting a "u up?" text in the middle of the night. A "good morning" text is a sweet way to show your partner that they were the first thing you thought of in the morning, and possibly the last thing you thought of before bed. And while a booty call text message is not inherently bad, they're not always received positively.
Everyone's relationships and communication styles are different. Some people aren't good at texting; they might be more touched by a video chat or a handwritten letter. And, of course, your relationship isn't contingent upon you sending a text first thing in the morning. But if you're looking for a sentimental way to tell your partner they're always on your mind, you can't go wrong with a "good morning" text.