section_path: Theories on function
section_name: Theories on function
paragraph_text: Theories on function
  
  For many humans across multiple eras and cultures, dreams are believed to have functioned as revealers of truths sourced during sleep from gods or other external entities. Ancient Egyptians believed that dreams were the best way to receive divine revelation, and thus they would induce (or "incubate") dreams. They went to sanctuaries and slept on special "dream beds" in hope of receiving advice, comfort, or healing from the gods. From a Darwinian perspective dreams would have to fulfill some kind of biological requirement, provide some benefit for natural selection to take place, or at least have no negative impact on fitness. Robert (1886), a physician from Hamburg, was the first who suggested that dreams are a need and that they have the function to erase (a) sensory impressions that were not fully worked up, and (b) ideas that were not fully developed during the day. In dreams, incomplete material is either removed (suppressed) or deepened and included into memory. Freud, whose dream studies focused on interpreting dreams, not explaining how or why humans dream, disputed Robert's 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.
  
  Theories of dream function since the identification of REM sleep include:
  
  Hobson's and McCarley's 1977 activation-synthesis hypothesis, which proposed "a functional role for dreaming sleep in promoting some aspect of the learning process...." In 2010 a Harvard study was published showing experimental evidence that dreams were correlated with improved learning.
  
  Crick's and Mitchison's 1983 " reverse learning " theory, which states that dreams are like the cleaning-up operations of computers when they are offline, removing (suppressing) parasitic nodes and other "junk" from the mind during sleep.
  
  Hartmann's 1995 proposal that dreams serve a "quasi-therapeutic" function, enabling the dreamer to process trauma in a safe place.
  
  Revonsuo's 2000 threat simulation hypothesis, whose premise is that during much of human evolution, physical and interpersonal threats were serious, giving reproductive advantage to those who survived them. Dreaming aided survival by replicating these threats and providing the dreamer with practice in dealing with them. In 2015, Revonsuo proposed social simulation theory, which describes dreams as a simulation for training social skills and bonds.
  
  Eagleman's and Vaughn's 2021 defensive activation theory, which says that, given the brain's neuroplasticity, dreams evolved as a visual hallucinatory activity during sleep's extended periods of darkness, busying the occipital lobe and thereby protecting it from possible appropriation by other, non-vision, sense operations.
  
  Erik Hoel proposes, based on artificial neural networks, that dreams prevent overfitting to past experiences; that is, they enable the dreamer to learn from novel situations.