The neurobiology of dreaming

What are dreams, and why do we have them? People have probably been asking these questions since the dawn of reflective thought, but it wasn’t until the 1950s that scientists first identified neurophysiological correlates of dreaming. A classic paper by Aserinsky and Kleitman1 in 1953 marked the discovery of what we now refer to as Rapid Eye Movement (REM) sleep (Figure 1). Together with non-Rapid Eye Movement (NREM) sleep, REM sleep if one of the two major sleep states that humans and other mammals pass through multiple times during each sleep episode. REM sleep is the state associated with the vivid, hallucinatory dream experiences that we (sometimes) remember after waking.


Sleep scientists typically monitor brain activity during sleep using electroencephalography (EEG), which measures large-scale electrical activity in the brain and can be used to determine the different stages of sleep that a human or animal subject passes through. If you record EEG activity from a normal, healthy adult, you will find that they begin a night's sleep by entering NREM sleep. They pass through four stages of increasingly deep NREM sleep before entering the first bout of REM sleep. As the night progresses, each successive bout of REM sleep becomes longer and each bout of NREM sleep becomes shorter, until will finally awaken from one last bout of REM (Figure 2a).2


The experiences that people report when awoken from NREM vs. REM sleep are very different (Figure 2b). In the original Aserinsky and Kleitman study, 20 individuals were studied several times each and, when awoken from REM sleep, "... 20 [out of 27 awakenings] revealed detailed dreams usually involving visual imagery; the replies to the remaining 7 queries included complete failure to recall, or else, 'the feeling of having dreamed,' but with inability to recollect any detail of the dream." In striking contrast to this, "Of 23 interrogations during [NREM sleep], 19 disclosed complete failure of recall, while the remaining 4 were evenly divided into the other two categories." 

The original 1953 study established that two very different brain states seemed to be occurring during normal sleep, and each was accompanied by very different patterns of brain activity, body movement, and conscious experience. While it is well-established that this is true, and that the vivid, bizarre dreams that we all often experience occur during REM sleep, it is still unknown and debated among neuroscientists what the normal biological function of dreaming is (if anything at all). In a future post, we will explore the biology of sleep and proposed functions of REM sleep and dreaming in more detail.


  1. Aserinsky, E. & Kleitman, N. Regularly Occurring Periods of Eye Motility, and Concomitant Phenomena, During Sleep. Science. 118(3062):273-4 (1953).
  2. Hobson, J.A. REM sleep and dreaming: towards a theory of protoconsiousness. Nature Reviews Neuroscience. 10(11):803-13 (2009).