i have no way of specifically answering your question because i know nothing about you or your medical history but one possibility would be stress or depression. in these cases your body may be at rest but your stressed brain may still be going 100 miles per hour.
why do i wake up exhausted after a full nights sleep? . i workout daily and i eat healthy. i know that i talk in my sleep. often times i'm aware of it when it's happening but i'm still asleep or dreaming it doesn't quite wake me up. could this have something to do with my exhaustion?