My dog barks at his own reflection in the mirror how do I stop this?

Every time my dog walks past the full length mirror in the bedroom he stops and barks at it. He's been doing this for 3 months, doesn't seem to be getting better on his own. He's a 2yr mixed breed. Is this a self-awareness thing? A territorial thing? And more importantly how do I get him to stop? The barking at 6am is not ideal.