Aggression Help?

My 4 year old dog only becomes aggressive at night. During the day he is very well behaved, but once it becomes about 6 or 7 o'clock it's like he's a whole new dog. Every time I move he runs over to protect things that are actually mine such as my books, papers, etc. or any of his stuff too. He's bitten holes in my shirt twice and my arm once. I don't know what to do. What do you think the problem could be? Please help. Thank You.

Asked by Member 1037035 on Dec 13th 2011 in Aggression