Formal:
* Je ne suis pas fatigué(e). (I am not tired.) This is the most common and literal translation.
* Je ne suis pas endormi(e). (I am not asleep.) This emphasizes that you're not falling asleep.
Informal:
* Je n'ai pas sommeil. (I don't have sleep.) This is a more natural and common way to express it in everyday conversations.
* Je suis réveillé(e). (I am awake.) This highlights the fact that you are alert and not tired.
Other options:
* Je suis en forme. (I am in good shape.) This implies you're energized and not sleepy.
* J'ai l'esprit clair. (I have a clear mind.) This suggests you're mentally sharp and not drowsy.
The best way to say it depends on the specific situation and your relationship with the person you're talking to.