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　　Have you ever dreamed of visiting a planet in the Milky Way(银河系)?While the trip sounds exciting, it would take years and years to reach your destination．So in the future, bedtime for astronauts may be more than a few hours of regular shut eye．They would have to sleep for years．

　　European researchers are now conducting hibernation experiments．The study may help them understand whether humans could ever sleep through the years it would take for a spaceflight to distant planets．“If there was an effective technology, it could make deep-space travel a reality,”said Mark Ayre of the European Space Agency last month．

　　What seems like a science fiction is not completely unlikely．Researchers have been able to use chemicals to put living cells into a sleep-like state where they don't age．They have now moved on to the small, non-hibernating mammals(哺乳动物)like rats．

　　A major challenge is the fact that cells can be very simple systems, whereas body organs are far more complex(复杂的)．“It's like moving from a simple Apple computer to a supercomputer,”said Marco Biggiogera, a hibernation researcher at Italy's University of Pavia．Just like bears and frogs, the hibernation of human beings would cause a person's metabolism(新陈代谢)to lower so they would need less energy．

　　Medical research, however, is just half of a spaceflight hibernation system．There is a challenge of designing a suitable protective shelter(栖身所)．Such a shelter would provide the proper environment for hibernation, such as the proper temperature．It would also have to monitor(监控)life functions and serve the physiological needs of the hibernator．

　　According to Ayre, the six-person Human Outer Planets Exploration Mission(使命)to Jupiter's moon(木星的卫星)Callisto, could be an opportunity to use human hibernation．The mission aims to send six persons on a five-year flight to Callisto, where they will spend 30 days, in 2045．