From the scariest nightmares to the most bizarre fantasies, some dreams are remembered forever, and a team of Italian scientists knows how. Researchers at L'Aquila and Bologna universities have determined that it's all about the theta waves. If those slow electrical oscillations are present in the cerebral cortex during the REM cycle, the dream is remembered. If not, it's lost forever. These same waves are at work in waking life, when we form an "episodic memory" of events.
"In short, we don't really know why we recall or forget dreams, but we have finally identified how we recall or forget them," says the lead researcher. Now go take a nap. (Read more dreams stories.)