That's what mine was like - nothing could wake him, including... crashing out of bed. At his age, that isn't normal. One way or another, I'd be concerned.
For us... it was mental exhaustion from a combo of dxes including Developmental Coordination Disorder (DCD) and Auditory Processing Disorders (APD), plus anxiety among others. Getting help for Developmental Coordination Disorder (DCD) (accommodations and Occupational Therapist (OT) therapy) and Auditory Processing Disorders (APD) (accommodations and technology) made a HUGE difference. I used to be able to clean his room while he slept. Now... I can't even tip-toe in to put away laundry!
