There can be a lot of causes for what you are seeing...but in scenarios that involve the inability for a cat to rest in an otherwise "healthy" lifestyle, is aging anxiety.
As a cat ages, the vision deteriorates and it's the night vision that goes first. It becomes more and more difficult for them to see when the lights are dim. With this decrease in vision you see an increase in anxiety and the cat will stop sleeping. They will try to engage you at all times because of their confusion. You can sometimes help this by using a night light. I would also want to get a very good ocular exam to rule out a real problem with the eyes.
Before you try drugs again run a few trials in the house. It might be hard to sleep but put a nightlight down in the socket that illuminates where the cat sleeps. Or try leaving on a bathroom light...but it needs to be more than just the casting of light from another room.
You might also want to try a homeopathic anxiety medication called homeopet (http://www.homeopet.com/). You can get it at most petsmart stores.
A simple anti-inflammatory might also help in case of arthritic pain.
Be sure and discuss all of these ideas with your vet so that you can get a plan in place to get you AND the cat some rest.
Let me know if you need more help.
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