We Americans have a confused and contradictory relationship with vacation. In theory, we love it; in practice, we often dread it. So much expectation is heaped on a few weeks of free time that disappointment, if not inevitable, is common. Worse, our escape from the job and daily routine fills us with anxiety that, somehow, this interlude will inflict a gruesome revenge once we return to work.

Nevertheless, we go forth

We hustle to beaches, mountains, national parks, theme parks, unfamiliar cities or (best yet!) the back yard. The democratization of recreation is one of the 20th century’s quiet upheavals. Leisure travel spending in 2015 totaled nearly $651 billion and involved 1.7 billion person-trips, says the U.S. Travel Association. In the 19th century, only the rich could abandon sweltering cities for cooler resorts: Saratoga, New York; Newport, Rhode Island; Cape May, New Jersey.

But some of us have trouble letting go. Millions of Americans aren’t taking all the vacation time they’ve earned — and that’s a commentary on America’s new work culture.

Here are the numbers: Among full-time American workers, about 90 percent receive some paid vacation, says the Labor Department. From 1978 to around 2000, these workers earned — and took — an average of 20 days a year of vacation, according to studies done by a travel industry group called Project Time Off. In 2015, full-time workers actually earned almost 22 days of vacation but took only 16. About half of workers leave some vacation days unused.

This amounts to 658 million unused days, worth about $223 billion in spending, the study says. Naturally, the travel industry tends to describe this as a crisis. It would prefer to see all that money going into the coffers of airlines, restaurants, theme parks and other tourism businesses. It regards people who don’t use all their vacation time as slightly deranged or worse. The industry’s term for employees who don’t exhaust their vacation days is “work martyr.”

You might think that this allergy to pleasure reflects the unsteady state of the U.S. labor market. People don’t spend too much time away from the office for fear that their jobs will have disappeared when they return. There may be something to this. After all, the gap between vacation-earned and vacation-taken first appeared around 2000, just as the tech boom was ending, and has gotten worse ever since.

But if the business cycle’s ups and downs mainly caused these anxieties, then the economy’s recovery (today’s unemployment rate: 4.7 percent) should have reversed them. It should encourage workers to use more of their vacation time. It hasn’t. What truly unnerves workers, argues study author Katie Denis, is growing Internet “connectivity,” from email to smartphones. The boundaries between work and home have blurred; the office is “omnipresent .