“Mind you, even if it turns out that the inhibition of prefrontal metabolism during REM sleep explains the disinhibition of dream content, it still doesn’t tell us anything about why some people’s brains would want to spend some REM time in a Busby Berkeley musical. The specific content of dreams remains a mystery. Moreover, if true, this speculation would constitute one of the classic features of science – in explaining something, you’ve merely redefined the unknown. If the answer to the question “Why is dream content so disinhibited?” turns out to be “Because prefrontal cortical regions are atypically inactive during REM sleep,” the new question obviously becomes “Then why are prefrontal cortical regions atypically inactive?””
Excerpt from “Why Are Dreams Dreamlike”, one of Sapolsky’s engaging and stimulating essays that make up Monkeyluv. The bolds are mine, though.