The guest room itself has, of course, disappeared -- in much the same way as the entrance foyer, the dining room and the attic. But the overnight guest has not.

Trundles, double-deckers and loft-type extra beds around in today's home-furnishings market. Most of these are inexpensive and easily meet a teen's need.

When it comes to adult sleep-overs, however, a different standard is required. A decent bed for one or two with a bit of space around it is the least of the requiremednts.

There are many sleep sofas available on the market. These wonderful extra sleep spaces require little room. You can make them up with sheets and blankets at the last minute. The only trouble with sleep sofas is the cost.

So, I'm always on the lookout for new ideas. I found just the thing I was looking for in the apartment for a newly married couple with an endless stream of friends in distant cities. Their need for guest space was crucial -- and a challenge -- in their new one-bedroom apartment.

It was obvious that the living room would have to be the guest room. They did not want the conventional sleep sofa, so I discovered the very thing -- an instant convertible.

The convertible, a chaise longue, is on a simple box-like base, upholstered to the floor. The tufted "mattress," actually the seating, is in the same brilliant malachite green cotton fabric, with a tiny dot for relief. One end adjusts so that the back can be raised for comfortable reading or conversion, or lowered to the horizontal position for sleeping.

Two are no more expensive than the average sleep sofa. The box of the base is hollow, so that most of the bedding, even comforters, can easily be stored within the bed structure itself -- a bonanza for any space-conscious apartment dweller.

I bought two chaises and placed one on either side of a greenhouse-type window. I used the vertical thrust of the glass much as I would have a fireplace, as a focal point for the room.

A large and spacious low wood coffee table provides plenty of surface for daily needs and doubles as a night table. At the foot of each chaise is an elegant chest, large enough to store pillows, linens and towels. Comfortable wicker armchairs flank the chest and table.

We painted the wall deep brown, the same color as the velvet carpet. A green abstract painting, plants, lamps and elegant flowers and accessories combine to make the room attractive by day, efficient by night.