Besides designing a spacecraft that can successfully land on and re-launch from Mars, scientists have had to consider one other hurdle to clear before manned flights to the planet are a possibility: It takes a really long time. A 17-month long Mars flight simulation has found sleeping pattern irregularities in the six "astronauts" confined in a capsule in Russia.

Scientists found that within a few weeks of starting the mission, several of the crew members developed sleeping disorders: Some of them started sleeping longer and being more sedentary during the day, another began sleeping shorter hours, and one crew member began operating on a 25-hour day.

Though concerning, lead researcher Mathias Basner, of the University of Pennsylvania School Of Medicine, says the disorders were relatively mild and would likely not pose a significant threat to any Mars mission.

"We saw large individual differences in how the crew responded," he says. "Overall, you have to say the mission was a success. There were no major adverse events. We observed that with certain individuals, there may be some issues. These need to be addressed before we venture on such a mission."

The 520-day mission length was chosen because that's about how long it would take to travel to Mars, perform a short mission, and return to Earth. "Astronauts" on the mission were subject to "a spaceship-like habitat; continuous isolation from Earth's environment; realistic mission activities; a mid-mission landing on a simulated Mars surface … communication delays inherent in interplanetary travel; limited consumable resources" and other hardships they'd be likely to experience on a Mars trip.

Basner says a future Mars-bound spacecraft could use advanced lighting techniques to simulate day-night cycles (the Russian simulation used only fluorescent lighting) and a crew could be carefully selected based on how they performed during on-Earth simulations. Sleep problems showed up just a couple months into the simulation, so Basner says shorter simulations could be used to weed out people who might be negatively affected by the environment.

"We don't have to put them through the 520 days, but maybe two to three months would work, just long enough to see bad coping habits going on in the subjects," he says.

The record for longest spaceflight is held by Russian cosmonaut Valeri Polyakov, who spent 437 days aboard the Mir space station between 1994 and 1995.

Besides the possibility of sleep disorders, recent studies have suggested space travel may cause more problems for the human body than previously thought. Last week, researchers at the University of Rochester said that iron radiation particles associated with space travel may cause early-onset Alzheimer's disease, and a study of 27 astronauts published in 2012 found brain and eye abnormalities caused by increased pressure in the brain. Space travel has also been shown to cause bone-density loss and muscle atrophy.

"Being less active is a potential problem," Basner says. "We know microgravity can cause bone and muscle loss—on top of that, if astronauts consciously decide to move around less, that may only aggravate the problem."