When she texts you first thing in the morning?

The hour she sent it tells you very little. Plenty of people text first thing because it is when they have a free minute with a coffee, not because they are signalling anything. What tells you more is the content. If she mentioned something specific about her morning, a bad night's sleep, a commute, a dog that woke her up, she is giving you a small thread to pull. If she sent a bare word with no context, she is likely checking in lightly and expects a light reply. Read which of these you are looking at before deciding how much to write. Overreading a "morning" into a serious message, or underreading a detailed one into a throwaway, are the two most common errors here.

When she gives you a detail, the most useful reply usually responds to that detail and adds one small thing of your own. You are not trying to match her length exactly, but you should be in the same register. If she is playful, be playful back. If she is tired and short, keep your reply short and warm. Add one specific thing about your morning or your day, something she could reasonably ask a follow up about. A reply with nothing of yours in it reads as polite but closed. A reply that dumps your whole morning on her reads as needy. Aim for one concrete detail and one question or hook she can take up if she wants to.

The point of a morning text is usually to make later contact feel natural, not to hold a full conversation before her first meeting. So close your reply in a way that does not demand a response right away. A light statement or a question she can answer in one line works best. Avoid anything that needs a long thought before she can reply, because she will likely put it off until she has time, and then the moment has passed. Let the thread sit comfortably. If she answers warmly, you can keep going. If she does not, you have still given her a nice start to her day without making her feel obliged.