Ah, summer, glorious summer. It's the season to loosen your collar, lighten your steps, pack your bags, and head off on vacation.

Or is it? Ask Stefanie Stadler, an account supervisor for a communications firm in McLean, Va., if she uses all her vacation every year and she replies with a quick No. "I do lose hours," she says.

But don't think Ms. Stadler's boss, Lisa Throckmorton, is playing Simon Legree and blocking the door. As Ms. Throckmorton explains, "Every week when I meet with her, I remind her of her paid-time-off balance."

It's a reminder Throckmorton, a senior vice president, could use herself. "I have 100 hours of vacation already this year," she says.

As both women forgo time they could be spending at the beach or in a European cafe, they have plenty of company. This year an estimated 51 million Americans – more than one-third of the workforce – will not use all their vacation days, according to a survey by Expedia.com. In what the company calls "vacation deprivation," each worker will pass up an average of three days off.

Other workers face a different challenge. Almost 1 in 4 Americans have no paid vacation and no paid holidays, according to the Center for Economic and Policy Research in Washington, D.C. In a new report, "No-Vacation Nation," the group notes that the US remains the only advanced country that does not guarantee workers a paid vacation. By law, Europeans have the right to at least 20 days of paid time off per year. Some countries guarantee 25 or 30 days.

Americans offer many reasons for not taking vacations. "For some workers, it is a show of loyalty to be at work all the time," says Jeffrey Pfeffer, professor of organizational behavior at Stanford University, in Palo Alto, Calif. "It shows your commitment and what a wonderful employee you are."

Some workers say they cannot leave because they have too much to do. Others can't afford a getaway or say they are too tired to plan one. Some want to save vacation time for emergencies. Still others don't want to come back to a heavy workload. Dual-career families find it especially hard to coordinate schedules. Some employees take their cues from those around them.

"A lot of people give up vacation days because they see their boss or co-workers giving up their days," says Noah Blumenthal, president of a coaching company on Long Island. "This creates a vicious cycle in which no one wants to be the first to take all their days, but everyone wants the culture to change."

This summer, with gas prices soaring, more Americans may choose shorter vacations closer to home. Some will simply stay home, taking what is called a "staycation."

For Stadler, who is in her mid-20s and single, giving up part of her time off is a matter of priorities.

"I'm ambitious," she says. "I look at the time I could be in the office helping clients. It's a trend of this age group. You don't want to be spending money when you might invest it, or spending time away when you don't necessarily need to. It's a life stage. I'll grow out of it. I love to travel."

Even the summer hours some companies offer, with half-days on Friday, can subtly alter vacation patterns. "I look at those and think that is like a mini-vacation," Stadler says. "I don't need a week at a time to refresh myself."

Throckmorton finds that the growing ranks of singles also have an effect. "We've got a lot of people in their mid-20s. Seemingly nobody in that age group has any vacation plans for the summer. I don't know if it's financial, or being single and not planning."

She counts herself among the nearly 20 percent of US adults who have canceled or postponed vacation plans because of work. "We rebranded our company in March," she says. "That was my responsibility. I put the company first."

She is also among the growing ranks who check work e-mail and voice mail while vacationing. Tethered to the office by "e-leashes," these employees are physically absent but still mentally present.

"Last year I went to Naples, Fla., for a week and sat on the beach with my BlackBerry," Throckmorton says. "It was probably more relaxing for me because I had a sense of what was going on at the office. A lot of people will say, 'My husband or wife won't let me take the BlackBerry.' I don't have anybody helping to set that parameter for me."

Brett West, vice president of a media-relations firm in Alexandria, Va., learned a vacation lesson the hard way. He stayed in the office the week between Christmas and New Year's, when the company was shut down, to help a client. Although he later received financial compensation from his boss, he says, "It's best to take the time that you're given for vacation or holidays. Make sure you've got vacation plans, and stick to them as much as you can without it being detrimental to your livelihood. You can never buy back those lost days. You can never buy back your family time."

This year Mr. West is taking his own advice. Two weeks ago, he returned from a trip to Germany, and he's already making plans for the holidays.

Phil Armstrong, managing director of a consulting firm in Washington, D.C., received "kind of a wake-up call" last month when the human resources department notified him that he must use nine days of vacation by the end of the fiscal year in June or forfeit the time. The company maintains a use-it-or-lose-it vacation policy.

Later in the summer, Mr. Armstrong and his family plan to spend a week or more on Martha's Vineyard off the coast of Massachusetts. Noting the advantages of longer stretches away from the office, he says, "On the rare occasion when I'm able to take two complete weeks, it's really the second week that's most enjoyable. It's when you adjust to the reality that you're not as indispensable as you think you are and get to the point when you can decompress."

That also benefits families. Referring to his two young children, Armstrong says, "They need more Dad time."

Most companies offering paid vacation specify the number of days. Fiberlink, a technology company in Blue Bell, Pa., has no formal policy for its 215 employees.

"They can take time off whenever they want," CEO Jim Sheward says. "We expect them to be responsible in not overburdening the team by taking excessive vacation. We don't even track it." The approach, he adds, is built on respect and the golden rule.

Vacationgoers and workplace ex­­perts alike attest to the restorative power of down time, with no alarm clocks, meetings, or rush-hour commutes.

Wallace Huffman, a professor of labor economics at Iowa State University, has studied men's and women's leisure time and its impact on productivity. He says, "Productivity could increase by up to 60 percent for employees in the month or two following a good vacation a week or two long."

Yet Pfeffer, the Stanford professor, believes that any major changes in vacation policy will require government action. "We'd have to look more like other industrialized countries, with a stronger mandate to provide paid vacation and limit work hours," he says.

Making a persuasive case for time off, Armstrong says, "Whatever you're doing on vacation, even if you spend an hour a day on e-mail, it's better than being in the office."