People may be learning while they're sleeping – an unconscious form of memory that is still not well understood, reported Kimberly Fenn, from Michigan State University (Michigan, USA), and colleagues.  The researchers studied a group of more than 250 people, finding that people derive vastly different effects from this "sleep memory" ability, with some memories improving dramatically and others not at all. This ability is a new, previously undefined form of memory. The team observes that: “there was a significant, positive correlation between [working memory capacity] and increase in memory performance after sleep but not after a period of wakefulness.”