"The first surreal sensations I had were that I was moving around in the tank, floating right to left, left to right, until I realised I was completely stationary"

When I did it the tank operator told me this happens because there are certain chemicals being released from your adrenal system, and that the direction you feel like you're spinning corresponds to the chemical being released. Not sure if that particular claim is true or not, but apparently there is a reasonably large body of academic research into the physiological effects/benefits of floating.

Also, Joe Rogan's video on the tank is a must watch:

http://www.youtube.com/watch?v=KeqmKwsvM58

interesting - I definitely felt like I was floating to the left the most

Not knowing if your eyes are open or closed is very strange. I experienced that once while splunking.

We sat in a cavern for a while with all of our flashlights off. At first you expect your eyes to start adjusting, then you realize that they aren't going to and you start to think that they're closed. After a while you quit caring what state they're in as it doesn't do any good. It's a very peaceful, almost serene feeling to (temporarily) have the information feed from major sensory organs cut off like that.

I've experienced the same when my bathroom light bulb goes out.

Anyone else had the strange experience when you wake in the middle of a night AND there is a power outage?

No blinking leds, no street lights: takes a few seconds to understand what is going on.