Formal:
* Lorsque je me réveille le matin: This is the most literal and formal way to say it.
* Au moment où je me réveille le matin: This is another formal option, emphasizing the specific moment of waking up.
Informal:
* Quand je me réveille le matin: This is a more casual way to say it.
* Le matin, quand je me réveille: This emphasizes the time of day, "in the morning."
Other options:
* À mon réveil: This literally translates to "at my awakening" and is a poetic way to say "when I wake up."
* Dès mon réveil: This means "as soon as I wake up."
The best option for you will depend on the context and your desired level of formality.