In a recent review of evolutionary theories concerning the possible adaptive function of dreaming, Barrett shrugs off the better-known psychoanalytic theories of dreams (for example, Freud's "wish fulfillment" and Jungian archetypes) as being irreconcilable with a Darwinian framework and instead highlights the major contemporary, biologically informed theories. Remember, the key question for us to consider is why dreaming occurs at all, since it's not immediately apparent why natural selection wouldn't have simply engineered a dreamless, non-REM sleep.