Night Aggression (Edited)?

My 4 year old dog is only aggressive at night. Does anyone have any idea why this might happen? He was a stray so we are thinking maybe it's a protective instinct or something...?? What do you think?

Edit: I was asked to give more details. So during the day my dog is well behaved. He listens pretty well and shows absolutely no aggression what so ever. He's just normal. Then around 9 o'clock he becomes easily angered and it's gotten to the point where if I even stand up and take a few steps or reach to grab something he will snarl at me. He's bitten me once and put holes in 3 or 4 of my shirts but the growling happens every night. I don't know how to stop this because I don't know what causes it. It's only at night which I don't understand. Please help!

Asked by Member 1037035 on Dec 31st 2011 in Aggression