How to treat separation anxiety in cats (and CALM HYPERACTIVE Cats as well)

Ok, so these are difficult problem, I am not going to lie to you. Although we know a lot about human psychology, we know little to nothing about the psychology of cats. Why does your cat have separation anxiety? Why is your cat chasing and terrorizing the other cats? There are various theories, but none of them are proven. As far as separation anxiety goes, it could be that your cat was separated from the mother to early; or it could be that your cat just loves you like nothing else and wants to be with you ALL the time (yes, like a stalker). And what about hyperactive cats? It could be simply their character. WHO KNWOS!? 
I know this is not very helpful so far, but the good news is that there are some thing you can do against these problems.

First of all there are only ways to treat the symptoms, not the anxiety or hyperactivity itself. And no matter what some people might tell you here, unless they can put the cat on the couch and start counseling in catish, nothing will cure the separation anxiety or hyperactivity if it is of psychological nature. I understand that it can be annoying if your cats if chasing others cats or is meowing in front of your bedroom door all night, so how can you stop that? 
Put on a Thundershirt.
I know, I know, you might think we all lost it, but give it a try! It works magic for many pet owners, why not for you? The thundershirt will calm your cat (or dog), and that is all you need to do. I am not sure what kind of magic this shirt does on cats, but as I am holding a degree in psychology I BELIEVE it has a lot to do with the pressure of the shirt giving the cat a feeling of security. Like a hug, or an arm around your shoulder from a loved one when facing emotionally challenging situations. Also, in psychology we sometimes use a so called hug machine. This machine is used to calm down hyper-sensitive persons. So I think you start to understand how this shirt works and where I am going with this.  

Are there any other treatment possibilities. Yes, but none of them are as simple as and cost effective as the Thundershirt. For example, you could take your cat to the vet and get medication, but this would be very pricy and in the end, taking medications for an extended period of time will not be very healthy for the cat. Comparing various treatments, I can only advice you to give the Thundershirt a try. It can help you sleep again and more importantly, it can help your cat be calm again.