More than two hundred years have passed since David Hartley (English psychologist and philosopher, 1705-1757) first proposed that dreaming might alter the strength of associative links between memories, and more than one hundred years since Sigmund Freud (Austrian neurologist and founder of psychoanalysis, 1856-1939) suggested that dreaming served to process traumatic memories. But it has only been since 1953, with the discovery of rapid eye movement (REM) sleep by Eugene Aserinsky and Nathaniel Kleitman, that studies of sleep's role in memory processing began in earnest. Since then, a wide range of studies have provided converging evidence on the important role sleep plays in the off-line reprocessing of waking memories. Whether dreaming plays a similar role remains unclear.

Sleep's complex role in memory consolidation makes our understanding of that role more difficult. Multiple memory systems exist within the brain store and process different types...