Monday morning.

The first day back on the job after two sun-filled, fun-filled weeks at the beach.

You're ready to plunge back into your work, full steam ahead. All that stored-up energy directed at new projects, unfurnished tasks. Right?

Very wrong.

If you're like hundreds, maybe thousands of Washington vacationers returning to work today, you're going to spend the next couple of days in a funk.

You're got the post-vacation blues, the end-of-the-good-times blahs. And it can strike equally at high-powered executives and the crew that cleans up the office overnight. Even if you really like your job.

You planned the trip for weeks, and the anticipation was part of the pleasure. While away, you forgot any nagging problems at home and work. Now you're back, and vacation is a whole year away again. That's depressing.

Mental health specialists see it frequently, though, says psychiatrist Terrence Chastek of the Psychiatric Institute of Washington, "It's not acutally a clinically recognized syndrome." Still, "we see it in our patients -- and we're all aware of it in ourselves and in our colleagues."

Look around the office today and you'll see the symptoms as described by those who have had them. Maybe you'll recognize them in yourself.

You can't focus your mind on the report you're reviewing. You've reread the same paragraph for a half-hour.

You daydream about that moonlight stroll at surf's edge the night when all seemed right with your world.

You spend several minutes seriously considering whether you can afford to move permanently to the beach, even if it means a substantial pay cut.

You visit the water-fountain twice as often as you usually do. You squirm in your chair a lot.

You spend much more time socializing, talking about your vacation to an ear that will listen.

You pick up a file folder with every intention of tackling it but phone a friend instead.

You mentally organize your slide show.

You're in a daze.

One young professional, back from two weeks abroad and facing a stack of projects, says, "I spent the first couple of days just organizing things. I kept making lists, cleaning my desk out, talking to people, telling them about my vacation.

Certainly, the condition isn't a serious one, and normally can be expected to disappear in two or three days, or a week at most. But it can hit hard.

Says one striving Washington couple just back from a grand camping trip in Alaska -- he a promising federal lawyer, she an energetic editor of a national association magazine:

"Awful, that first week back was awful. You wouldn't believe it." They eased the reentry by editing 20 rolls of film into a manageable slide show for friends within a few days of their return.

The longer you are away, it seems, the longer the recovery.

A Washingtonian able to get out of the capital for eight weeks each summer appeared to be suffering a full week of the blahs. Struggling to get his mind back on his work, he sighed:

"I'm always glad when the plane lands at the beginning of my trip -- because I know if it crashes on the way back, at least I've had my vacation."

Why do we experience the post-vacation blues?

It could be the holiday was especially relaxing, and our self-discipline got out of practice. It takes a day or two back at the desk to shape it up. The trip may have been too hectic, and we're just worn out. Or it was so pleasant, the feeling lingers a day or two more.

For some people, suggests Leander Wick of the Mt. Vernon Center for Community Mental Health, "When they return, they no longer have vacation as an escape defense from their problems. They're back to an I'm caught feeling."

People "need to be realistic about vacations," he says. "They can't give you a new lease on life."

John Neulinger, professor of psychology at City College of New York, and author of "The Psychology of Leisure," considers the reentry period a problem of "leisure lack." He defines leisure as "doing what you want to do for its own sake."

"If you take a person who experiences leisure during his vacation -- he does what he really likes to do -- then he's returning home in a state of deprivation." For those who get that kind of leisure on vacation but not on their jobs, "the contrast" on that first Monday back "will really get you."

Neulinger, who spends his summer months in upstate New York, has his own reentry problem. "Here I am looking out at a barn, apple trees and green grass. I don't look forward to going back to New York City. Have you ever been in New York City?"

The Psychiatric Institute's Chastek sees these other causes for the blues:

Fear of not being needed : You return from your trip and find "the organization has survived. Everything is going as usual. You can't see evidence that your absence had any impact."

Performance anxiety : Some people can't function as well for a day or two. They're not sure of themselves." A secretary has trouble typing, for example. Clients have described, he says, how for a day or two they have "forgotten what they needed to do in the job."

The remainder of lost childhood : Vacations, he says, tell us that we are "growing up and aging." They "stir up memories of those long, leisurely summers of swimming, riding a bike, softball" of childhood and remind us we have grown up and lost the pleasureable experience."

Our tendency to be self-critical :Some people, he has found, feel "they should never have left their responsibilities, that they didn't deserve to have such a good time." Others even feel "that someone -- the boss or superior -- is going to be angry because they weren't there."

Fear of being replaced : Most vulnerable are workers in "particularly competitive jobs in high-pressure organizations," says Chastek. Patients have shown up "quite anxious" about colleagues who filled in for them while they were gone. The over-50s worry about "a junior replacing them."

Being reminded of how important our jobs are to us : At vacation's end, we may see ourselves not as fulfilled as we might have expected, and may even be ready for the vacation to be over. It tells us, he says, "how important our jobs are to us -- to our self-identity. It may be depressing that we need them so much.

Nevertheless, says Chastek, "considering how hard we work," the post-vacation blues "is not a reason not to take a vacation."