You've heard the stories: late at night a man is awakened in a state of unexpected anxiety, to find himself suddenly unable to move.  

Weird, humanoid-looking figures stand around, looking on while the terrified fellow is caught in an eerie state of immobility. Sometimes fantastical events follow: he is taken away from his home, shown strange sights, even flown through outer space. Could this really be alien abduction?

While it wouldn't be fair to completely rule out those aliens, modern sleep-researchers are more familiar with these same kinds of experiences under a different name: sleep paralysis.

Sleep paralysis is a recognized form of sleeping disorder that often affects young people, but can occur at any point in life. It's a condition whereby the normal sleep cycle is interrupted and the sleeper arises to a state of partial consciousness, rather than being either completely awake or asleep. In other words, the brain regains sensory awareness, but motor control remains "off line."

The half-and-half mental state of sleep paralysis is often associated with a sense of creeping anxiety as well as strong visual, auditory, and even olfactory hallucinations. It seems that people with this disorder are partially awakened during what would have been a dream episode -- the contents of which may then invade consciousness as a series of disturbing visions. To top it off, not infrequently there is a deluded sense of being viewed, or even controlled, by some unseen entity.

All of this matches closely with many modern-day UFO abduction scenarios. So, while it isn't as exciting as a visit from space, sleep paralysis may in fact provide a more down-to-earth explanation for those close encounters that come in the night.