As summer approaches, millions of people around the country are strategizing about how best to spend their time off.

They're doing so, experts say, at a time when work and family demands are escalating, squeezing the amount of vacation time that people take and intensifying their expectations of it. Many see vacations as a chance not only to relax but also to learn about themselves, meditate on the direction of their lives and address personal problems, surveys show. And as often as not they return disappointed -- feeling as much in need of time off as before.

The psychological needs of vacationers can go unsatisfied, whether during a bicycle trip through New England, tropical week on the beach or holiday break at Grandma's. Researchers who study leisure have interviewed thousands of tourists of all ages, analyzed travel diaries and vacation memories, and joined tour groups to discover what sours a vacation.

Travel snafus aside, they say, frustrated tourists usually have no one to blame but themselves -- either because of too-high expectations, because they've revised the memories of past vacations or because the vacations simply didn't fit their needs.

"Socrates said it more than two thousand years ago, but it absolutely applies when you take time away: Know thyself," said Andrew Yiannakis, a University of Connecticut sociologist who studies personality and vacation choices. "As hard as people are working now, it's crucial to think about what the time can provide and what it can't."

Short-term benefits

When taken regularly through a working life, time off can be good medicine -- physically and emotionally. In a 2000 study of 12,338 middle-aged men at risk for heart disease, researchers at the State University of New York in Oswego found that those who did not take regular vacations were more likely to die over a nine-year period than those who did, especially from heart problems.

Several smaller workplace studies confirm the short-term benefits. People tend to sleep better after more than a week off, have fewer physical complaints than they did before the break, and report being more optimistic and energetic than they were before.

These effects may last five days or five weeks -- and depend on how satisfying the break was, researchers believe.

Vacation satisfaction is a hard thing to measure or describe, but psychologists say one component is simply the sense of being outside our usual roles, of experimenting with different identities, however tentatively. Careen Yarnal, a Pennsylvania State University researcher, has interviewed and traveled with an informal group of about 100 people, most middle-aged, who take an annual vacation together on a cruise liner.

"One of the things they find most satisfying is simply being able to behave differently than they usually do," she said. "It's little things: They dress differently, wear risque dresses or loud shirts, they say things they wouldn't say back home. They overindulge."

The need for a break seems greater than ever. American workers today put in three and a half hours more per week than they did in the 1970s, according to the Families and Work Institute, a nonprofit research center in New York that conducts ongoing national surveys. Couples, with or without children, report working about 10 hours more a week combined then they did in the 1970s.

In a 2001 study, the institute found that 26% of workers do not use all their yearly vacation time, usually because of job demands. And a 2002 survey of 1,893 tourists found that 3% of them report feeling such responsibility to their jobs that they have headaches, fatigue and nausea when they're away for more than a few days -- a phenomenon the Dutch psychologist Ad Vingerhoets calls "leisure sickness," a combination of guilt about being away and dread of what crises are filling the in-box.

Any experienced vacationer knows it can take a few days to shake this sickness and mentally take leave. But after that, psychologists say, most people not only expect relief and invigoration -- they insist on it, regardless of how they actually feel during the vacation.

In a recent series of experiments, a team of investigators led by Northwestern University psychologist Leigh Thompson followed three groups of vacationers, interviewing them before, during and after their time off. One was a group of 21 men and women on a guided 12-day tour of Europe; another included 77 students on a five-day Thanksgiving break; and the third was 38 young adults on a three-week bicycle trip in California who kept diaries.

The pattern was the same in all three: excited anticipation, followed by disappointment in many cases. Weeks after the vacation, some still felt let down. But most members of each group found that their recollections were re-created as warm memory. "We call it rosy retrospection, and it is particularly significant" after vacations, said Thompson. "People need to have good things happen, it's such an important break, so they're constantly reviving their view of events, even if it was pouring rain the whole time. It follows a general psychological principle: getting what you want by revising what you had."

This revision serves a useful psychological purpose. It reassures us that we have defied the daily grind and are not wholly defined by it. But it also unconsciously heightens expectations for the next vacation, and the risk for disappointment, Thompson said. Exhausted and anxious after a week of family tension in Kauai? In time, memory tends to turn the trip into a slice of paradise. A year later, you're likely to be planning the same kind of trip, with high expectation and the same potential for conflict.

No one has done a careful study of what people plan to do with perhaps the most precious commodity promised by vacation: time to think. Many people say they can find real clarity when looking at their lives from the outside while being caressed by warm surf and chilled Mai Tais. But psychologists also have amassed considerable research demonstrating how a windfall of empty hours can turn free-floating anxiety into the darkest rumination.

"We often set up unreasonable expectations for repairing relationships, changing the direction of our lives, having deep, meaningful conversations with our spouses, and that's just asking for trouble," said Susan Nolen-Hoeksema, a psychologist at the University of Michigan who studies the link between rumination and depression.

Too much time to think

A conversation at the swim-up bar may indeed help restore a relationship.

"The risk is that if things don't go as planned, and people have time to think, instead of problem solving they begin to ponder one difficult issue, which activates thoughts of another, and then there you are at the pool suddenly feeling your life is out of control."

Needless to say, jet lag and hangovers don't help. Sitting wide awake and alone at 3 a.m. with a splitting headache and a vague sense that your life is unraveling is one of the universal treats of foreign travel.

Nolen-Hoeksema, who was planning a Florida vacation when interviewed, said she had spent time identifying the things that on past holidays had triggered family conflict and hours of tense rumination -- and planned boat rides, museum visits and other activities specifically to skirt those problems.

"The idea is not to avoid thinking through important issues altogether," she said. "Just don't expect to be able or willing to do this the entire day."