No two dreams look the same, but their underlying neurology is similar. “The brain activity, it looks like it’s awake,” explained Bruno van Swinderen, a neuroscientist at the University of Queensland who studies consciousness and the function of sleep. Dreaming appears to be a distinct process with neurological and physiological responses, such as brain activity and bodily movement. 

          Profile photograph of a white man in a blue, red, and yellow shirt.
Bruno van Swinderen studies the functions of sleep at the University of Queensland. 
University of Queensland

Scientists observed these types of actions across animal models using electrical brain probes and cameras to capture eye movement and other bodily changes. From flies to octopuses, they found evidence that these animals dream.1,2 If researchers can observe dream-like behaviors in so many animals, it begs the question, what is the smallest animal to dream?

To van Swinderen, this comes back to the function of dreaming, which he studies as a mechanism for anticipating and interpreting the world in which one lives. “The evolution of active sleep or the evolution of dreaming is fundamentally linked to the evolution of a capacity to pay attention,” he said. Animals that move in their environments and must react to environmental changes have to be able to make predictions. van Swinderen suspects that dreaming helps the brain practice this. 

For example, scientists used calcium imaging to detect neuron activation in sleeping mice  and found that it mirrored that of feeding behaviors.3 This neural activity appears to be tied to complicated brain structures. “The animals that don’t have [these brain structures] are the ones that basically don’t really have a brain,” van Swinderen said. These would include nematodes and jellyfish but exclude arthropods like insects and crustaceans. Considering that arthropods, such as the commonly researched fruit flies and jumping spiders, are on the scale of a few millimeters, dreaming animals can get quite small.4  

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  1. Yap MHW, et al. Nat Commun. 2017;8:1815
  2. Medeiros SLDS, et al. iScience. 2021;24(4):102223
  3. Oesch LT, et al. Proc Natl Acad Sci. 2020;117(32):19590-19598
  4. Röβler DC, et al. Proc Natl Acad Sci. 2022;119(3):e2204754119