Over the last couple of months I've noticed a habit whereby my dog will snap at other dogs, but only at night. She will be lying there, not asleep, but just lying there and if another dog gets too close to her she will snap at them. She doesn't just do it when she's lying down and dogs are too close - she at times has literally run across the room to snap at our one dog in particular. Tonight she snapped at this same dog while they were on their way into their beds in the bedroom.

But she ONLY does it at night. It's not an encouraged behaviour, and the one dog she does it to is submissive so she just lets her "bite" her face (she doesn't actually chomp down), and moves on, but the problem is the dog she went for last night is a smaller dog who gets into a panic and it turns into a full blown fight if I don't anticipate what she's doing fast enough (like tonight, because my back was turned and I didn't see the warning signs to intercept).

The little dog has cancer, terminal, and I wonder if that's why she goes her for a lot? She doesn't make a move on our other, even smaller, dog so it's not about dog size.

I plan on getting some calming products for her at night, but I wish I understood where this behaviour comes from. We have a giant garden and she plays a lot with the submissive dog, and I give them lots of attention. I can't seem to pinpoint exactly when she first did it. We rarely (and I mean really rarely) go out at night so I can't imagine anything traumatic happened to her at night... She is an older dog (10 years old in a couple of weeks, she's a large breed dog) so maybe it's old age anxiety?

@shelbi thank you. She is at the vet every 3 or 4 months for shots (she has hip dysplasia so she gets an anti-inflammatory shots), and they always check eyes, teeth, temp, etc every time. I'll bring this up next time because they're saying she's in perfect health otherwise (the new vet at the practice thought she was 3 years old the way she was carrying on).