Source: http://gizmodo.com/what-is-it-like-livin...

The first thing you notice is that everything seems really heavy. After 95 days on the International Space Station, I returned to Earth in the Space Shuttle Discovery. I took off my helmet and it felt like I was holding the anchor of the U.S.S. Nimitz in my hand. Oh great, I thought, how am I ever going to brush my teeth — the brush will be too heavy!

The next thing you notice is that your vestibular system is all messed up. Just sitting up took a lot of concentration. After about 15 minutes I was able to stand, but I would have keeled right over if there weren't a lot of things to hold onto in the middeck of Discovery.

You see, your brain is remarkably adaptable. After just a few days in space it figures out that your inner ear is producing nothing but garbage signals and so the brain turns the gains of those signals way down in its Kalman filter and cranks up the gains on your visual sensors, the eyes.

Then all is well, until you come back to Earth. Now you need your inner ear sensor again, but the brain is still filtering it out. Gradually the brain re-calibrates, but it takes a while.