"Most clients don't know I've gone away," said Black, vice president of Chesapeake Closets, whose duties range from securing new business to approving designs to updating the company Facebook page. "I typically don't consider any day an off-day. I am not comfortable being cut off."

Black typifies a growing number of American workers who can't seem to put down their BlackBerrys, laptops and iPads and disconnect - even if they're sitting on a beach or sailing the seas aboard a cruise ship. And many of those who vow to unplug and get away from it all often return from vacation burned out, never really managing to decompress.

A survey released recently by Expedia.com showed that 55 percent of workers come back from time off without feeling rejuvenated, and others struggle to cope with work-related stress while they're away. In a separate survey, staffing firm Robert Half International found that 69 percent of financial executives check in with the office at least once or twice a week while on vacation.

Americans are not only working more during office hours but also paying more attention to their jobs during vacations, according to workplace experts and recent studies.

"It used to be that work was like a belly button: You were either in or out," said Alan Langlieb, a Baltimore psychiatrist and business consultant and former director of Workplace Psychiatry at the Johns Hopkins University. "And now, for most people who work, they're always in."

Langlieb added that "technology allows you to be at work 24 hours a day anyplace in the world." He said: "You end up seeing nonstop work where there's never really an off period."

The line between work and leisure has been blurring for years, a trend exacerbated by the recession and global economic pressures, experts say. Staffing cuts have left heavier workloads for employees who remain. And some workers, worried about layoffs, believe they have to forgo breaks or vacations.

"Staffs have been cut pretty thin, and there's more on each person's plate to handle day in and day out," said Steve Saah, director of permanent placement services for Robert Half in the Washington area. "When times are good ... it's easier to step back. But on the heels of the recession, there is more work that needs to get done by a smaller pool of people."

Though American workers were given four more vacation days this year than last - an average of 17 days total - 37 percent said they will not take all of their vacation days this year, up from 34 percent last year, according to Expedia's 10th annual "vacation deprivation" survey.

Nearly half of the 1,000 respondents surveyed by Expedia reported working more than 40 hours a week. And nearly a third said they check work e-mail or voice mail while vacationing, up from nearly a quarter last year, Expedia said.

Executives are particularly prone to staying plugged in, according to a separate survey of 1,400 chief financial officers at U.S. companies.

More than two-thirds of participants in the Robert Half Management Resources Survey released last month said they check in with the office at least once or twice a week during their vacation. Of those who check in, about one-quarter do so several times a day.

Candice Bennett heard an earful from friends and colleagues about taking her work along on her honeymoon last year, a four-day trip to Barcelona and a Mediterranean cruise.

"I took my laptop. I took my cell phone and told everyone, 'You're going to have to text me, and then I'll call you.' And I was checking e-mail," said Bennett, 34, who runs a marketing and research company based in Lorton, Va., that does surveys for companies such as Microsoft and Choice Hotels.

"I was getting lectured, 'It's your honeymoon. Why are you on the computer? You should be out,'" she said. "I try to be unplugged, but I never fully succeed."

Bennett said she works about 60 hours a week, driven in large part by a business that operates 24 hours a day, with phone surveys at night and clients all over the world. But she said her overachieving work ethic also is part of her personality.

"There's a little bit of anxiety that goes along with not being in the loop," Bennett said. "Even when I was working for another company, I didn't want to miss something on a client I was working on. ... And I don't want to miss an opportunity for my business."

Workplace experts say it's fine to work hard, as long as workers set boundaries for themselves and balance work with other parts of their lives. Problems can arise if workers can't do those things, which can lead to medical problems, anxiety or depression, or damage to personal relationships.

Bryan E. Robinson, a psychotherapist with a private practice in Asheville, N.C., who has treated workers for stress and burnout, said even before the downturn he had a steady stream of clients.

Some of his clients were employees of large banks who were afraid to take lunch breaks for fear of not being seen as team players. Others told him it's not worth it to take time off because "you work double hard to get ready to go and double hard when you come back, and you're worried the whole time because 'I'm getting behind,'" he said.

"There is a rampant fear in this country that someone might be angling for your job, and if you take time off, someone else will be perceived as more effective or more loyal to the company," said Robinson, author of the 2007 book "Chained to Your Desk."

"The downturn has made a big difference; it has really scared people," he added. "If they see people being laid off, they are not willing to take that risk" of taking a break.

Work can develop into an addiction for people who can't turn it off, he said, and an abrupt disruption to constant work can lead to "leisure sickness," with physical symptoms such as fatigue, muscle pain, agitation, mild depression or anxiety.

Langlieb said the line between "just working hard" and when work habits are actually a disorder depends on the person. "There are plenty of people who get tremendous job satisfaction ... from always being needed and always being available," he said, "and then others, if you start raising the dial too high ... will burn out."

Julie Lenzer Kirk, who teaches entrepreneurship to mid-career women who are trying to start technology businesses through a program at the University of Maryland, Baltimore County, said she often works until 1 a.m. or 2 a.m. Even when she's home watching a movie with her teenage daughters, she's likely to have her laptop in front of her.

"I've been known to answer e-mail at 3 in the morning," said Lenzer Kirk, who also recently started a nonprofit to help women entrepreneurs, which she now runs from her home. "There's just so much to do, and I'm passionate about what I do. If it's hard for me to sleep because I'm thinking about it, instead of lying there thinking about it, I get up and do something."

She had her first nonworking weekend in a long time on Mother's Day, when she and a friend took a trip to Key West, Fla.

"I did not take my laptop," she said. "I did have my BlackBerry, and I'm sure I responded to a couple e-mails, but it was not as intense. It was a huge accomplishment."