I don't think that most people would consider lying in a dark, soundproof box for over an hour to be relaxing. But, I'm not most people.
I recently tried an isolation chamber, aka "float" tank, for the first time. A float tank allows the participant to come as close as possible to not experiencing any of their physical senses for a prolonged period of time (hence another name, "sensory deprivation chamber"). The the tank allows virtually no light or sound and has a shallow pool of highly dense salt water that keeps a human body afloat, permitting you to float while touching, seeing, or hearing anything. The goal is that, once inside for a while, you feel like you are disconnected from your senses while you float in nothing.
People have various motivations for using float tanks. Using them for just 40 minutes can alleviate stress, allow the body to heal injuries more efficiently, allow muscles to relax, and provide a other skin and edge case medical benefits. Some people, like me, just find the idea relaxing. Most people I talk to don't think they would enjoy the experience (and they're probably right), but I'm a very introverted person and I spend much more time inside my head than out of it. When I'm thinking I find external stimuli to be distracting, too much of it can be annoying or even tiring. I enjoy having quiet time with very little stimulation, and a float tank is the quietest session you can have. When I found commercial sensory deprivation chambers being marketed as flotation tanks I was instantly intrigued and bid my time until I had the chance to try one.
I went to a local salon and spa, which offered a couple of float tanks among its services. Float tanks aren't too easy to find, but after some searching around it seems that most metropolises have at least one place that offers them. The float tank itself was essentially a large, covered bathtub in a small, dark room just a couple feet wider and longer than the tank itself. The procedure was to shower, enter the tank, close the hatch behind yourself, and an attendant would knock on the room's door once the time was up (taking further measures to wake you if necessary). I opted not not use background music (recommended) and to float in the nude (to avoid feeling any clothing, also recommended).
Closing the hatch behind me for the first time felt odd. I can't say I've ever stepped into a small box with no practically no light or sound. There was a sudden rush as I could almost feel the light and sound leaving my brain and I was suddenly very aware of how much of both I had been processing just before closing the hatch. The tank was virtually sound proof; I couldn't hear anything; no hallway chatter, no honking cars, no footsteps, nothing.
The Physical Aspect
The water was just one foot deep, but the extreme salt density made that plenty to keep me afloat. I extended my arms and legs to touch the sides of the tank and center myself, then pulled my limbs slowly off the side to let myself sit motionless in the middle. This was tricker than it may sound, since the slightest bit of momentum can cause drift and eventually touching the sides. I had to try a couple times to succeed.
At that point I experienced a very unique feeling. I saw nothing, heard nothing, and felt almost nothing. The water was body temperature and whenever I was motionless for an extended period of time the water feeling would subside to being minimally noticeable. But any movement or conscious thought about it would allow me to feel it. The feeling wasn't distracting by any means, but it was still a sensory connection to outside world.
One of the keys to floating is to relax as much as possible, both mentally and physically. Relaxing physically was actually a bit trickier than I had expected. I tend to be somewhat highly strung internally and I often tense muscles without realizing it. (I believe most people do this to some extent or another.) It took longer and more focus than I expected to relax all my muscles. After about 5 minutes I realized that I still had some of my facial muscles on the left side of my mouth tensed slightly, a little later I realized my right quadriceps were a little tensed, yet later I realized I had re-tensed my face, etc.
After about 20 to 30 minutes of being completely motionless it felt like my muscles were almost dead. While I knew I could move any muscle I wanted to, it felt like it would require tremendous effort to do so. At one point I twitched my foot, just for fun. It felt like there was a 10 pound force working against my foot as I twitched it. I think it may have produced some muscular benefits, since I felt several brief localized muscle spasms that were possibly tight muscles relaxing.
The sensation of lying in the tank was nothing like lying in bed. For one thing, my posture, suspended in the water on my back, let my head sit farther back than it would if I were lying on a normal hard surface. Initially it was a bizarre feeling, since it felt like my head was sitting too far back and of my control, but I got used to it. The rest of my body was held in a perfectly comfortable floating equilibrium. You can still feel a bed, the sheets feel soft, the mattress offers firm, albeit ignorable, resistance. The float tank offered no sensation or feeling. It wasn't snuggly, warm, or just kind of quiet. It felt like as close to nothing as possible. (Interestingly, tests have shown that replacing dense water with a bed does not provide the same benefits.)
The Mental Aspect
Once I was centered and relaxed I was very comfortable and felt completely alone with my thoughts. I let my mind wander for some of the time, and I let myself focus my thinking for other times. Aside from my own heartbeat, it kind of felt like time stopped.
My brain felt so unencumbered while thinking. It was like a CPU able to run a dedicated process without interruption from I/O and other processes seeking time-share. When they were focused, my thoughts were in one of those extremely laser-like grooves that come along only occasionally. I had compiled a general list of things to think about in the tank before hand, and without going into specifics they covered various ideas from my programming projects to philosophical quandaries. I was able to analyze and organize things very quickly, and had extra time to pursue other ideas that came up I felt no time pressure while thinking, I moved from step to step as I felt comfortable doing so.
My biggest motivations for using the float tank was relaxation along with my thoughts. I was not disappointed.
Unfortunately, I did make a mistake. At some point early on I instinctively touched my face with my hand, probably to scratch an itch. At about the 15 minute mark I opened my eyes, just to see how much light there was in the tank now that my eyes were adjusted (answer: almost none, I could barely make out the walls 2 to 3 feet away). That allowed some very salty water to run into my eyes. I was doomed because I couldn't get it out of my eyes with my salt-water covered body. I tried ignoring it, but after my eyes burned for 20 seconds I gave in. I exited the tank, wiped my face off with a towel, got some water from the shower and cleaned my face and thoroughly flushed my eyes, and got back in. The problem took only a minute to fix, but it was still a disruption.
I ended up floating for a total of 1 hour 45 minutes. That's a long time to be without any physical stimulation, but I really enjoyed it. I came out of it very relaxed and feeling pretty good. I didn't even fall asleep once, although I had kind of expected to. I would do it again.
Advice to Potential Floaters
Based on my experience, here is what I would offer to anyone planning to try float session.
Spend a minute in the beginning getting yourself positioned. - You don't want to touch any of the sides of the float tank. Unfortunately, any bit of momentum causes you to drift, and if you start drifting you likely will probably bump into a side. Any time I made any noticeable movement I extended my arms and legs to the side until they touched the sides, used them to center myself, then slowly withdrew them.
Avoid getting salt water in your eyes. - This may seem extremely obvious, but it's worth emphasizing. Don't even get salt water on your face, and avoid opening your eyes regardless.
Spend some time focusing on relaxing your muscles. - I think that focusing on your body too much would defeat part of the purpose of floating, but it's worth spending some time up front intentionally relaxing all your muscles. It may not be as easy as lying down and telling yourself to relax. I'd recommend spending 5 or so minutes just focusing on relaxing every muscle from your face to your toes. It's very easy to tense them unintentionally.
How do you know if you would enjoy floating? It's probably impossible to know short of actually doing it, but here's a pseudo-test to screen out some who definitely would not like it: Take a pair of the best earmuffs or headphones you can find, put them on, and lie on a bed in a dark room without a pillow for five minutes. If you feel like ending before the time is up, you would probably not enjoy it. If you find it relaxing, floating may be enjoyable for you. (I enjoy doing that sort of thing, that's why I was fairly certain I would enjoy a float tank.)