A creepy one I found
A recent study by the National Psychiatric Institute in Boston, Massachusetts, concluded that no activity can account for the phenomenon known as nightmares.
Whereas many dreams come from unconscious desires, most nightmares seem to come from an outside source independent of the individual. In fact, when subjects are asked to recall nightmares they are almost always found in the same memory section as actual physical memories, not the section where normal dreams are replayed.
So, in other words, those aliens and creatures you see at night in your “dreams”?