Joshua James Houle claimed he had a history of waking up violently, and says he warned Misty Lynn Ward of that.
When she tried to wake him after a night of drinking, he used a nearby knife to stab Ward.
The court heard that in his panic, Houle feared the police would not believe his story so he tried to dismember her body in the bathtub of his apartment.
Ward was found by police after blood and water collected in a storage room beneath Houle's apartment.
He will be sentenced Jan. 29.