General synonyms:
* Conscious
* Alert
* Vigilant
* Aroused
* Aware
* Asleep (This is technically the opposite, but can be used in a contrastive sense)
More specific synonyms:
* Wide awake (emphasizing being fully awake)
* On the alert (emphasizing readiness)
* Rested (after sleeping)
* Bright-eyed (suggesting energy and alertness)
Figurative synonyms:
* Alive (suggesting being mentally active)
* Attentive (focused and paying attention)
* Responsive (reacting to stimuli)
The best synonym to use will depend on the specific meaning you want to convey.