Awaken can be transitive or intransitive. The opposite of the transitive verb, as in "I awakened you" would be lull to sleep. "I lulled you to sleep." The opposite of the intransitive version, "You awakened" is to fall asleep- "You fell asleep." Its not really that hard; don't we use these terms almost daily? Though i usually say "I woke up" rather than "I awakened," which seems a little precious and not what i would teach my esl students.