The Mars-500 project was series of psychological simulations of the long journey to Mars that took place in 2007 and 2011. The 105 days simulation and the 520 days simulation , that started on June 3rd 2010 and ended on Nov 5th 2011.
The 520 day mission ended on time. There were no interpersonal conflicts noted, no any events which required interrupting the experiment. Cultural & language differences of the 3 Russian, 2 European, & 1 Chinese crew did not cause difficulty. (All spoke some amount of English, but not all spoke Russian).
The crew were at one Earth gravity during the whole trip rather than at Mars' Low gravity. However, many Mars missions expect to provide artificial gravity by swinging the Mars spacecraft on a long teether (anchored to the third stage booster to act as counter mass), to provide either Earth's gravity or Mars's gravity during the transfer orbit to or from Mars. Since Mars missions expect to use artificial gravity during the trip to and from Mars, the lack of zero gravity does not invalidate this experiment.
As boredom set in, the crew spent more time in bed, most notably during the 'trip home'. At the end of the experiment, 4 of the 6 crew suffered from sleep problems, and 1 person suffered from psychological issues. 
- SSC RF - Institute of biomedical problems of RAS
- Проект «Марс-500» (Russian home page of the project)
- [https://en.wikipedia.org/wiki/MARS-500 - wikipedia article on this experiment.