Sleep problems common on simulated space mission

Here is an article about an experiment done to simulate space travel. Astronauts were confined to a pressurized spacecraft in Moscow for 520 days straight. Their sleep was monitored with an actigraph, a type of accelerometer. After a month, some of the crew members began to have sleep problems. It seems that one of the issues is that the low light levels on the spacecraft are not strong enough to simulate the day-night cycle we experience daily. Since our brain's clock depends on light to keep our rhythms regular, exposure to stronger light sources on the spaceship may help in actual space flight.