leisure sickness
n. Ill health that occurs during leisure time, especially on weekends and vacations.
Leisure sickness, like paradise syndrome, chronic fatigue syndrome, seasonal affective disorder and sick building syndrome, is one of those modern conditions that sounds bogus, the pointless invention of some under-employed researcher, unless you happen to suffer from it, that is.

How do you know if you have been struck down with leisure sickness? Well, think back to the past few times you were ill. Did you have to take time off work? Or, after a prolonged period of good health, were you struck down by some mystery bug just as you were setting off on a long-awaited two-week vacation to some far-flung corner of the world? If it is the latter, chances are you, too, have leisure sickness.
—Sally Weale, “Do you often feel ill on holiday: but never when you're at work? If so, you could be a victim of 'leisure sickness',” The Guardian, November 26, 2002
2001 (earliest)
Some individuals can easily work a 12-hour day, but suffer symptoms such as headaches, muscle pain and fatigue during weekends or vacations, said Dutch scientists. They said this "leisure sickness" arises from the stress some people experience when they are unable to give up control and relax.
—“Some people suffer from leisure time,” Investor's Business Daily, March 12, 2001
Freelancers in the crowd may have noticed a puzzling phenomenon in their lives over the years: that within a day or two of completing a major project they become sick with a cold or other illness. This has happened to me more times than I could shake a stick at (if, that is, I was the stick-shaking type). I always assumed that my body was gamely fighting off infection while the deadline loomed, and then let things go once the project was safely in the can. Now I'm wondering if I was succumbing to leisure sickness.

This phrase was coined by Dutch psychologists Ad Vingerhoets and Maaike van Huijgevoort, who presented a paper titled "Leisure sickness: An explorative study" at a meeting of the American Psychosomatic Society on March 7, 2001. Symptoms include headaches (even migraines), fatigue, muscular aches and pains, and illnesses such as colds and flus. Sufferers (and for about 3% of the population, this occurs every weekend), typically have, according to the authors, an "inability to transition from the work to the non-work environment, a high need for achievement and a high sense of responsibility." Or they may simply have a work-friendly immune system.