Such studies you should take with a grain of salt. Still, I found these results interesting:
Jeroen Nawijn of NHTV Breda University of Applied Sciences in the Netherlands found a holiday happiness curve: Our mood tends to be lowest through the first 10 percent of a holiday and quite high during the “core phase,” which spans about 70 percent of the vacation time. Our spirits soar on the day before going home.
…Philip Pearce of James Cook University in Australia studied tourists visiting tropical islands along the Great Barrier Reef and discovered that their moods were particularly negative on the second and third days of their holidays, the time during which they also seemed to develop the most health problems. These ailments included skin rashes, tiredness, allergies, ear infections and asthma.
Yet it is not just a new climate or cultural differences that can make you feel bad; it is also the free time itself. Ad Vingerhoets, a quality-of-life expert at Tilburg University in the Netherlands, calls this a “leisure sickness.” People with this condition develop symptoms of illness during weekends and vacations, even though they rarely feel bad at work, he says.