Freud, whose dream studies focused on interpreting dreams, not explaining how or why humans dream, disputed Roberts hypothesis and proposed that dreams preserve sleep by representing as fulfilled those wishes that otherwise would awaken the dreamer. Freud wrote that dreams "serve the purpose of prolonging sleep instead of waking up. Dreams are the GUARDIANS of sleep and not its disturbers." A turning point in theorizing about dream function came in 1953, when Science published the Aserinsky and Kleitman paper establishing REM sleep as a distinct phase of sleep and linking dreams to REM sleep. Until and even after publication of the Solms 2000 paper that certified the separability of REM sleep and dream phenomena, many studies purporting to uncover the function of dreams have in fact been studying not dreams but measurable REM sleep.
The brain activity capable of formulating such dreams, rare among literate people in later eras, conforms to the bicameral mentality hypothesized by Julian Jaynes as dominant into the second or first millennium BCE. Framing the dream experience varies across cultures as well as through time. Dreaming and sleep are intertwined. Dreams occur mainly in the rapid-eye movement (REM) stage of sleep—when brain activity is high and resembles that of being awake. Because REM sleep is detectable in many species, and because research suggests that all mammals experience REM, linking dreams to REM sleep has led to conjectures that animals dream. However, humans dream during non-REM sleep, also, and not all REM awakenings elicit dream reports. To be studied, a dream must first be reduced to a verbal report, which is an account of the subjects memory of the dream, not the subjects dream experience itself. So, dreaming by non-humans is currently unprovable, as is dreaming by human fetuses and pre-verbal infants. Subjective experience
Preserved writings from early Mediterranean civilizations indicate a relatively abrupt change in subjective dream experience between Bronze Age antiquity and the beginnings of the classical era.In visitation dreams reported in ancient writings, dreamers were largely passive in their dreams, and visual content served primarily to frame authoritative auditory messaging. Gudea, the king of the Sumerian city-state of Lagash (reigned c. 2144–2124 BCE), rebuilt the temple of Ningirsu as the result of a dream in which he was told to do so.