At its worst, it can consume your life.

* "When you wake up, it's beautiful and sunny out and you have 33 zillion things to do. But instead you think, 'How am I going to get through this day? What do I hang on to?' " says Marie, a graduate school student.

For others, it can strike on a milder note.

"You're wishing you had someone to share an experience with, but there's a kind of paralysis," says Maxine, a 47-year-old writer. "That's when it takes discipline to pick up the phone and call someone."

Either way, it hurts.

"I went for a really long walk -- about 30 miles -- one Saturday on the C&O Canal," says Dennis, a 42-year-old consultant. "I passed a lot of couples and started thinking, 'Gee, it would be fun to do this with someone else.'

"There's a feeling of emptiness, like there's a second half of you that's missing."

Researchers and mental health professionals are recognizing that loneliness is more widespread and important than once thought. New studies are providing a link with physical health, while estimates of the lonely reach as high as 50 percent of the population.

"It's symptomatic of a screwed up society that doesn't provide opportunities for people to interact, or have places to go where they're comfortable," says Stephen Goldston, a psychologist with the Neuropsychiatric Institute at UCLA. "There's nothing that exists in communities like the student union at college, where you can meet people you're interested in without pressure."

Loneliness, he adds, is more than not having a date on Saturday night: "It's a key factor on people's physical and mental health."

After days so bad she wondered how she would live through them, Marie decided to seek therapy.

"It's painful, and a lot of times you don't really know why," she says. "It's not that there aren't lots of people around you, or that you're not outgoing and social, but inside you feel different. To recognize that is to feel lonely."

'People Are Ashamed'

It's not the easiest topic to bring up.

"People are ashamed of loneliness," says Jeffrey Young, an assistant professor of clinical psychology at Columbia University. "Even books addressed to lonely people don't use the word in the title."

Says Dennis: "I think you can feel you're failing if you're by yourself -- and in a sense you are, because you're not taking the steps needed to get out of it."

Marie, in her mid-twenties, is a model student -- a self-described overachiever who loves sports, music, socializing.

"If you have all this, how do you tell someone that you're lonely?" she asks. "You go out of your way to hide this part of yourself. You don't deal with it."

Loneliness is also tied up with machismo: Men aren't supposed to need anyone. Anne Peplau, a professor of psychology at UCLA, notes that it seems to be easier for women to admit to loneliness.

"If a survey uses the word 'loneliness,' more women than men say they are," she says. "But if the question is, 'I wish I had more friends' or 'My relationships are too superficial,' men score equally high."

Research is turning up other surprises. The elderly have been portrayed as the loneliest group, but surveys are instead turning up the late teen-age years -- 17 to 19 -- as the worst.

"The teen-age years are times of social upheaval, when you leave your family and go out in the world. As you get older, there are fewer major transitions, and you also tend to develop more social skills," explains Peplau.

Another possible reason: "People today who are 60 and 70 are of a generation that just doesn't complain. So teen-agers today may still be saying they're lonely when they're 50."

Meanwhile, interaction studies at the University of Tulsa suggest that while lonely people are clearly less verbally skillful and socially adroit, they are nevertheless not overwhelmingly rejected by strangers. Instead, it's the lonely who do the snubbing.

"They expect their partners to reject them, but by and large they do not," says Warren Jones, a Tulsa professor of psychology. "It's almost as if lonely people fear rejection so much that, before someone could possibly reject them, they reject the other person. If you think no one loves you, it's easy to decide no one's love is worth anything."

'Snap Out of It!'

Nearly everyone who's gone away to school, ended a relationship or moved to another city has been lonely. The admonitions delivered by parents, friends or self-help manuals -- "snap out of it!" or "get out of the house and meet some people!" -- are well-meaning but often poorly received.

"The problem is, people who answer that way are showing they're different from you, and that they don't have the problem," says Dennis. "What you really need to be reminded of is that there are a lot of other people who are lonely."

For most people, the so-called "garden-variety" of loneliness is transient, gone as soon as they make friends at the water cooler or the grocery store. However, there's a minority for whom it is a continual companion.

"The cliche's are good advice for most, but there's a subgroup for whom they don't apply," says UCLA's Peplau. "For them, loneliness is not a temporary state but a fact of life."

In the typical profile, these people had a parent who was unloving or absent. The person then ends up either constantly fearing he'll be rejected, or he avoids relationships altogether.

Family problems brought out her loneliness, says Marie. "Basically, the emotions were locked into a tiny chamber. These things catch up to you. All it took was something to trigger it."

Where a parent consistently favors another sibling, a feeling of mistrust, rejection or inferiority can develop, says Young of Columbia. Later in life, the individual will tend to anticipate that his partner will find someone better.

"The usual pattern is to then put your partner in a testing situation, constantly saying 'Do you really love me? Why did you say that? Pay attention to me rather than him.' " The result: The partner is driven away, and the lonely person is reassured that he's unlovable.

Seeking the Future Perfect

There's someone out there who's just right for you. She may be in your school, office or apartment building, or she may only exist in your dreams. She's the one whom it will take to make your life perfect and complete. And even if she doesn't know or care about you, you're not going to settle for second best.

This fantasy -- which women can also have about men -- is one many chronically lonely people have, researchers say.

"If you ask lonely people what their relationships ought to be," says Jones of Tulsa, "they will often tell you a very elaborate, detailed Hollywood version, with bells ringing and violins playing -- no ups and downs but only ups. Yet the terrain over which a relationship goes is much more varied than that."

*Dennis, while agreeing that it's good advice not to live your life as if your happiness were dependent on finding the perfect person, argues that "if you deny that it exists, you'll never find it. But if you look for it, you might end up with more than you expected."

The key, says Dave Burns, a Philadelphia psychiatrist and author of Intimate Connections (Signet, $4.50), is to practice on the less than perfect, the second-best.

"Once someone wants you, then someone else will, and then others. And once you get some experience, and learn the art of getting people to chase you, then -- when that special person comes along -- you'll have the skills and self-confidence to make a connection."

A search for that special someone may be further handicapped by conditions beyond your control. Workers on the night shift can find their lives on a totally different schedule from their dayside friends. A Mormon in a Baptist town, a Georgian in Maine, or an American in Japan may feel equally isolated.

"The kind of job we want, the place we live, how much money we have, how much free time -- all can have severe consequences for our social life," Peplau says.

Women tend to suffer the most from this.

"There's a widespread belief in our society that women should marry men who are not only older but have a higher income and better education," she says. "Women at the top of their professions must either reject that notion or find a partner from another field."

*Dressing for Success

Certain abilities -- how to flirt, listen, carry on a conversation or give compliments, how to call someone for a date -- are acquired when you're young. If you didn't learn these skills as a teen-ager, the researchers say, there's a greater chance of chronic loneliness.

On the bright side, says Jones of Tulsa, "is a number of therapeutic strategies. Research we have done suggests that just about everything works, from social skills training to group therapy to traditional psychoanalysis."

The bad news: "The lonelier the person is, the more difficult it is to get them into therapy or a counseling center."

For those who only suffer from intermittent loneliness, the advice of the professionals comes back to finding activities where you can test out your social skills. It's a safe way of playing it, because you're not putting yourself on the line by going into a bar.

Burns offers himself as test case: "I was lonely up to the age of about 26. Luckily, a friend taught me how to dress more appealingly, to make conversation, flirt, smile."

In other words, he covered up his real nature and turned himself into a phony. It's a charge he cheerfully accepts.

" 'I'm glad there's a real you,' I tell patients who protest. 'I got rid of mine several years ago and I've been feeling better ever since.' The idea that 'I have to be the real me' can be a form of rigidity and selfishness."

If you present yourself in an appealing way, he adds, "You're saying, 'I like myself, I like you, and I'm going to dress in a way that's going to turn you on.'"

On the Downhill Side

The first two-year results from a study of 3,000 Iowans over 65 -- about 80 percent of the men and women in that age group in the counties of Iowa and Washington -- found that the loneliest were five times as likely to be institutionalized. They're also four times as likely to die.

*"The easiest answer would be that these people die because they're unhappy and don't want to go on living," says Dan Russell at the University of Iowa's Center for Health Services Research. "But since the study controlled for depression, it appears there's something unique to loneliness that influences mortality."

Supporting research at Ohio State University is showing loneliness could weaken the immune system, the body's defense against infectious and malignant diseases. In studies of medical students and psychiatric inpatients, those who labeled themselves as lonely had lower levels of natural killer cell activity -- an anti-tumor and anti-viral defense.

"If you're lonelier, you're more distressed," says Janice Kiecolt-Glaser, associate professor of psychiatry. "After several days, your immune system may be poorer and, at least hypothetically, the risk of illness from something like flu is greater."

There's no long-term data yet, but "studies show that people who are widowed, separated or divorced have a higher mortality rate from certain infectious diseases and cancer. And these groups are certainly likely to be lonelier."

An ongoing AIDS study by psychologist Thomas Coates at the University of California, San Francisco, is also showing a relationship between psycho-social factors (including stress, depression and loneliness) and a weakened immune system. The connection holds true both for those who have and have not been exposed to the AIDS virus.

*"If you reduce stress and improve the quality of social relationships, your immune system may be strengthened, and thereby helped to withstand the virus," says Coates. "The study adds scientific weight to advice by health care providers to take care of yourself."

Building the Bridge

Loneliness has one good point: It signals that needs aren't being met.

"It's like hunger; it tells you to go out and get some nutrients, that there's a demand that isn't being met," says Columbia's Young. "But it can go too far. Just as starvation is not useful, so can extended loneliness be very damaging."

And despite the myths that people ascribe to love and friendship, it may be a much more mechanical process, like building a bridge.

"It's hard. You can't just sit back and let everything magically fall into place, as it often does in movies and books," says Jones of Tulsa. "And you have to repair it from time to time."

To focus on constructing a magnificent relationship may also be a mistake, he says. "You don't have to build the Golden Gate. Before you decide on an idealized version of what ought to be there, you have to get your feet wet."

"The problem is really lack of responsibility for one's life," says Maxine. "Loneliness is a misnomer for laziness, for stopping rather than going forward."

"It starts," says Dennis, "with the recognition that other people are lonely too."