This is a typical futon on the
floor. The futons and bedding are put away during the day in big wall closets called oshiire, and taken out at night. This clears the space for other use during the day.
But increasingly people prefer to sleep on beds. This is an ad for a mattress.
The Seinfeld episode is probably referring to capsule hotels - http://en.wikipedia.org/w
This is a report on a very sleek, modern version of the capsule hotel. The setup is still typical though: shower and bathing facilities separate, a locker provided for each guest, and a small capsule to sleep in.
Here's another video about the same capsule hotel. (Most capsule hotels have an image of being run down and a bit grotty, so this place is an exception. It would be nice to see such places crop up everywhere though. )
A more recent phenomenon is the napping salon, where busy workers can go to take power naps. Some capsule hotels are also open during the day for taking naps or resting.