To anyone who has worn contact lenses, advertisements for the new "extended-wear" models sound appealing -- if not irresistible. Imagine being able to fall asleep while watching Johnny Carson, reading the alarm clock immediately upon awakening or enjoying an overnight without dragging along cleaning solutions and a storage case.

Composed of a special plastic one wearer likens to "a tiny piece of Saran Wrap," extended-wear lenses have a high water content that permits necessary oxygen to reach the cornea, even during sleep. They can be worn during any activity -- except swimming or in the presence of noxious fumes -- and need be removed only for cleaning, usually once every two weeks.

In the 20 months since the Food and Drug Administration approved their use for general vision correction (they were approved in 1979 for cataract patients), consumer interest in extended-wear lenses, say several area eye-care specialists, has been tremendous. Of the two companies with FDA approval for manufacturing "general use" extended-wear lenses, Hydrocurve II estimates about 200,000 wearers and Permalens claims half a million.

"The rate of growth has been staggering," says L. Dean Clements of Cooper Labs, makers of Permalens. "Over the last five months, sales have been growing at a monthly rate in excess of 20 percent."

But despite the excitement, "the new extended-wear contact lenses are not exactly as carefree as consumers are led to believe," cautions Frank Fontana, a St. Louis optometrist who chairs the American Optometric Association's Contact Lens Section.

"Extended-wear lenses can be a great advantage, especially for people in certain occupations such as firefighters and physicians, but as with other contact lenses, problems can develop if they are not properly fitted and if wear and care regimens are not followed."

Although the lenses are similar to daily-wear soft contacts, he says, they are "even more fragile" and must be handled "with great care." Once in the eye they are difficult to dislodge, but can sometimes slip off center. "Wearers have to be careful not to rub their eyes," he says, "which usually involves learning to sponge rather than wash eye lids." Also, eye drops are often necessary for comfortable wear.

"There are two major points the lens advertisements don't make clear," claims Chevy Chase ophthalmologist Neil Martin, who did a study of the long-term effects of extended-wear contacts last year with Dr. Walter J. Stark of Johns Hopkins' Wilmer Ophthalmological Institute. "First, very careful follow-up is needed. You need a basic eye exam to decide if the eye is healthy enough and if your vision can be corrected by extended-wear lenses.

"Fitting itself can take a couple of hours, since you must sit for a half-hour or more with the lens in to see how your eye tolerates it. If you go through several lenses, it can take all morning. Then you must return the following day to be checked, after which you need to come in every week for a month, then monthly for four months, depending on how you're doing."

Secondly, he says, extended-wear lenses "aren't for everyone. It's easy to abuse them if you're lazy, and that can result in lots of nuisance infections. Many people can leave them in for months, even though the recommendation is to take them out for cleaning overnight once every two weeks. Any time the eye gets red--which could be from infection or tightness--the wearer must be checked by a specialist."

Any problems caused by extended-wear lenses, however, are usually reversible, notes Martin, whose study of 106 nearsighted patients who had successfully worn extended-wear lenses for four to eight years showed no cases of permanent visual loss. (Since that time he has heard of one case in which a patient was hospitalized as the result of an infection associated with use of the extended-wear lens.)

Although "not without risk," says Martin, "extended-wear lenses are still a valuable alternative for patients seeking a surgical correction of myopia." The radial keratotomy operation involves making several cuts in the eye, a procedure "the National Advisory Eye Council recently stressed is experimental and has not been subjected to adequate scientific scrutiny."

The key to success with extended-wear lenses, says Martin, is "careful patient selection." The optimum candidate, he says, has healthy eyes with sufficient tears and no allergies, "moderate to high" myopia (nearsightedness) and little or no astigmatism, is bright, highly motivated and has good hygiene habits.

For those who meet these qualifications, the extended-wear lenses can be a welcome substitute for glasses or daily-wear lenses.

"They're the greatest thing that ever happened," claims Peggy Wright, a 43-year-old Department of Defense employe who started wearing them in 1978 as a "test patient" prior to FDA approval. "I'd tried other lenses and couldn't wear them. I like to take a nap when I come home from work and conventional lenses were a real pain."

The major advantage, she says, "is that you don't have to worry about them. In 1980 I broke my neck in an auto accident and I had the lenses on for two months and I had no problem. I can even swim in them by wearing goggles."

"For me they're a godsend," says Washington optometrist Stephen Glasser, who was one of the investigators for an extended-wear lens company and has been wearing them himself for one year. "My vision couldn't be corrected with standard hard or soft lenses. At this point there's no way I could go back to glasses."

"It's nice to be able to wake up and see," agrees Robert Doyle, 35, a Department of Labor employe who got his lenses 15 months ago. "I've had no problems, except one time I had some redness and needed something to clean them with."

The biggest problem patients report, says optometrist Fontana, is "the replacement cost. I've had no one with (non-cataract) extended-wear lenses last more than one year."

Replacement is usually necessary, he says, as a result of tearing or build-up on the lenses. Consumers also should ask about the doctor's refund policy in case they are unable to wear the lenses, says Fontana, who has had "about an 80 percent success rate" with extended-wear contacts.

"There are definitely some improvements in the works," he says. "They're working on an ultra-thin, but more rugged extended-wear lens that may be available within the next six months. If someone is considering them, but is unsure, I don't think they're unwise to wait six months to a year."