for over a year.
Valeri Vladimirovich Polyakov (Russian: Валерий Владимирович Поляков, born Valeri Ivanovich Korshunov on April 27, 1942) is a former Russian cosmonaut. He holds the record for the longest spaceflight in human history, staying aboard the Mir space station for more than 14 months (437 days 18 hours) during one trip.<1> His combined space experience is more than 22 months.<2>
Selected as a cosmonaut in 1972, Polyakov made his first flight into space aboard Soyuz TM-6 in 1988. He returned to Earth 240 days later aboard TM-7. Polyakov completed his second flight into space in 1994–1995, spending 437 days in space between launching on Soyuz TM-18 and landing on TM-20, setting the record for the longest time continuously spent in space by an individual in human history.<2>
http://en.wikipedia.org/wiki/Valeri_Polyakov It is uncertain whether the low (one-sixth g) gravity on the Moon is strong enough to prevent detrimental effects to human health in the long term. Exposure to weightlessness over month-long periods has been demonstrated to cause deterioration of physiological systems, such as loss of bone and muscle mass and a depressed immune system. Similar effects could occur in a low-gravity environment, although virtually all research into the health effects of low gravity has been limited to zero gravity.
http://en.wikipedia.org/wiki/Colonization_of_the_Moon But you do mention a serious concern.
Gravity in Spaceby Tega Jessa on August 10, 2009One of the big challenges to long term travel in space is gravity. We now know that other than radiation and the cold vacuum of space, the biggest threat to a person’s health is lack of gravity. The human body was developed to thrive under the force of gravity. This is why our muscular and skeletal systems are shaped the way they are. Over time, in a zero g or weightless environment, the bones of the body become brittle and certain muscles, like those in the thigh actually weaken. There are also worries about its effect on the circulatory system as well.
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So scientist and even science fiction writers have been thinking about how to create gravity in space. There are several proposals some already proven and others that are still theory. The first is to us the centripetal force of a rotating hull. This is method is the most trustworthy as the effect has been observed on Earth. Every object in the universe wants to go in a straight line. In the case of planets, its the pull of gravity from an object with great mass that curves most celestial object’s paths. In the case of a space craft it would be the hull itself. Since you can’t walk through solid walls the force of your body wanting to go in a straight line would resemble gravity.
This theory has drawbacks. First, the amount of gravity you feel would vary depending on how close you are to the center of the craft. Scientists believe that the longer the radius of the spacecraft the less the effect will be felt. The other challenge is nausea. You are still basically on something like a huge merry go round. Like on its namesake, some people will be able to handle it and others will experience nausea. But in most cases its believed that if if the space craft rotates at 1 rotation per minute the nausea won’t be an issue.
The other method of maintaining gravity is to constantly accelerate a spacecraft at 9.8 m/s, Earth’s acceleration due to gravity. The problem with this is that due to limitations imposed by fuel there is no spacecraft that can maintain this rate of acceleration more than 7 minutes. This idea might become possible if a more reliable source of propulsion is found.
http://www.universetoday.com/37091/gravity-in-space/ 
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