Vacation time can undo a lot of mental stress. (Philip and Karen Smith / Getty Images)

True, it's hard to put a positive spin on the situation if you're standing at the end of a mile-long line and your flight is due to take off in five minutes. But it may be possible to perceive an irritating seat mate as a chance to hone your social skills or lost luggage as an excuse to spiff up your wardrobe.

The post-vacation brain

Some studies with rats have shown that stress can actually shrink parts of their brains, and a 2009 study in the Proceedings of the National Academy of Sciences found that it probably shrinks your brain too.

But not to panic!

The rat studies found that three weeks of restraint-induced stress led to shrinkage of tree-like projections called dendrites in a part of the brain called the medial prefrontal cortex — and impaired performance on an attention task. Importantly, though, these chronic stress effects disappeared once the rats had been stress-free for four weeks.

In the parallel human study, medical students who'd spent a month preparing for stage one of the medical board exam were tested on an attention-shifting task as close to the day of their medical exam as possible. Then they were tested again a month after the test, when their sources of stress were way down.

They too performed relatively poorly the first time they did the task, when they were badly stressed, and much better — just as well as a control group — after an essentially stress-free month.

The researchers could not, of course, get direct evidence that there was dendrite shrinkage in the med students that was subsequently reversed. But they inferred that this happened, since the results of every other aspect of the studies were so parallel, says study lead author Dr. Conor Liston, a research fellow in the department of psychology at Weill Cornell Medical College in New York.

How long a vacation?

"How long it takes you to relax after a stressful period of time depends on how quickly you can reset your perspective on life," says George Slavich, an assistant professor at the UCLA Cousins Center for Psychoneuroimmunology. "Taking a relaxing trip to Hawaii can help, but it's not necessary, nor is it always sufficient. No tropical vacation can help if the stress is mostly in your head."

On the other hand, Pruessner says, when you're on vacation, you probably have more time than usual for self-reflective thought, which could help you recognize that your stress is mostly in your head — and maybe even work on getting it out of there.

In the medical student study, a month was enough to make stress-impaired brains get their groove back. But wait! Don't snort. Sure, a month off is inconceivable. But it's possible that less time could do the trick too.

Pruessner once did a simple experiment with a group that had come for a two-week meditation-and-yoga retreat at a hotel in Canada. He measured blood levels of the stress hormone, cortisol, twice: at the beginning of their stay and again at the end.

At the beginning, the group could be divided into three sets: those with normal cortisol levels, those with higher than normal levels and those with lower than normal levels. Pruessner inferred that the first set was not particularly stressed, the second set was somewhat stressed and the third was even more stressed. (Scientists believe that when stress gets really, really bad, the body's cortisol response can sometimes stop working.)

At the end of the two weeks, those in the first set still had normal cortisol levels; those in the second set had lower levels than before, implying that they had become less stressed; and those in the third set showed no change, implying that their blunted response systems had not improved.

Pruessner's interpretation? Two weeks of rest and meditation and yoga were enough to help those who were somewhat stressed but not enough to help those who were more stressed than that.

"Ideally, you don't want to wait too long to take a vacation," he says. "You don't want to get to that third stage."