Sudden fighting between cats

Question
Hi, I have a fixed 1 1/2 year old Siamese mix male who I've had since he was eight weeks old.When he was about 10 months old I decided to get him a friend and adopted a fixed female who was also about 10 months old.They adapted to eachother within 2 days.Now, 8 months later, they start fighting at random times during the night.This is very new behavior for them.Usually, all they do is play and give each other baths, but lately all they do is fight at night so we have to separate them.For some reason this behavior is only happening at night, during the day its back to being friends.My cats are indoors cats only, but recently there have been 4 stray cats outside and I was wondering if maybe that has something to do with it.The fighting has really been cutting into my sleep. Please help with advice if you can.I would greatly appreciate it. Thank you.

Answer
Hi Christina,

You've answered your own question. Yes, the stray cats outside will bring out behavior like you describe between your two cats. They are fighting at night because one or more of the strays is getting very close to their home. Since they can't get at the offending cat they attack each other. It's called Redirected Aggression.

The easiest way to take care of this problem is to take mothballs and spread them around the perimeter of your property, (if you can). This will keep the cats away. If you can't spread them around the perimeter at least do it near any windows or doors that your cats can see out of. If they can't see them they shouldn't have a problem. Good luck.

Ciao, Karen