Cassiopea jellyfish prove that sleep requires no brain

By Armando Ortiz ’19

Cassiopea xamachana, a species belonging to the Cassiopea genus, is a primitive jellyfish with no brain that raises questions about the physiological origins of sleep. Source: Wikimedia Commons.

Cassiopea xamachana, a species belonging to the Cassiopea genus, is a primitive jellyfish with no brain that raises questions about the physiological origins of sleep. Source: Wikimedia Commons.

The mechanisms of sleep and its effects on the nervous system continue to baffle scientists with many unanswered questions surrounding this topic. One recent study on primitive jellyfish further adds to scientists’ questions about the origin of sleep cycles, and whether the presence of a brain is necessary for it to occur.

Scientists in California Institute of Technology recently discovered that Cassiopea, a primitive form of jellyfish, experience sleep despite the absence of a brain (1). Scientists from Caltech used three criteria in assessing the presence of a sleep cycle in these primitive jellyfish. The criteria for sleep, they explained, assessed whether there was a period of reduced activity, a decrease in response to the surrounding environment, and an increased urge to sleep during normally active periods when sleep-deprived (1,2).

At nighttime, Caltech scientists observed how Cassiopea reduced the number of times the jellyfish pulsated from 58 times per minute to 29 times per minute, which signaled a sign of reduced activity (1). Additionally, Cassiopea’s reaction time to scientists disturbing its resting platform was delayed compared to its normal reaction time during the day, demonstrating a reduced awareness of its surroundings (1). Finally, scientists disturbed Cassiopea sleep by pulsating the water every 10 seconds for 20 minutes during nighttime, which resulted in Cassiopea sleeping during the day, thus meeting the last criterion for sleep (1).

Scientists traditionally thought of sleep as a basis for restoring optimal functions within the brain, a mechanism controlled and dictated by various subsections in an astonishingly complex system (2). Yet this Cassiopea study contradicts this perception of the complexity of sleep, as the study demonstrates that sleep can occur at the neuronal level without the need for a brain. This further adds to questions about where exactly does sleep occur, and what is its evolutionary history.

Researchers Ravi Allada and Jerome M. Siegel from Northwestern University and UCLA respectively explained that sleep is beneficial as a restorative process necessary for optimal function in the nervous system (2). The evolutionary benefits of sleep, however, are paradoxical, as observed in the Cassiopea study. During sleep, organisms are more vulnerable to predators due to their decreased awareness of their surroundings, yet despite this disadvantage, organisms could perish easily without it (2).

One possibility that Allada and Siegel mention is that sleep in its simplest form consists of restoring neural circuit performance. The complicated control mechanisms that require the brain emerged from evolution as a way to efficiently restore neural circuit performance in more complex organisms with complex behaviors (2). The implications of the Cassiopea study, nonetheless, provide insight into the physiological origins of sleep, and raises questions about how sleep’s evolutionary advantages allow for it to remain an integral characteristic of the descendants of primitive organisms such as Cassiopea.


  1. California Institute of Technology. (2017, September 21). Jellyfish, with no brains, still seem to sleep: Jellyfish and humans may seem wildly different, but both still need to sleep. ScienceDaily. Retrieved September 23, 2017 from
  2. Allada, R., & Siegel, J.M. (2008). Unearthing the phylogenetic roots of sleep. Current Biology, 18(15), 670-679. Retrieved September 23, 2017 from doi: 10.1016/j.cub.2008.06.033
  3. Ravi D. Nath, Claire N. Bedbrook, Michael J. Abrams, Ty Basinger, Justin S. Bois, David A. Prober, Paul W. Sternberg, Viviana Gradinaru, Lea Goentoro. The Jellyfish Cassiopea Exhibits a Sleep-like StateCurrent Biology, 2017 DOI: 10.1016/j.cub.2017.08.014