A study of jumping spiders provides the most comprehensive confirmation so far that spiders sleep. Researchers found that Portia fimbriata enters two distinct sleep stages, one of which resembles rapid eye movement (REM) sleep in humans, a phase commonly associated with dreaming.
Published October 26 in the Proceedings of the National Academy of Sciences, the study used infrared cameras to record the spiders continuously through the night. During daylight, the animals remained alert even when stationary, turning, scanning their surroundings and adjusting their posture in response to visual stimuli.
As evening approached, the spiders searched for places to rest, typically hanging upside down from silk platforms after grooming themselves. They then gradually drooped during the night, entering a phase the researchers called quiescence. This was interrupted by short episodes involving body twitches, curled legs and rapid eye movements, which the team identified as signs of REM-like sleep.
The researchers also tested how quickly the spiders responded to a loud sound at different points in their daily cycle. Awake spiders reacted in an average of 1.69 seconds. Their responses slowed to about 2.71 seconds during quiescence and 4.27 seconds during the REM-like phase, supporting the interpretation that the animals were asleep rather than simply inactive.
The findings build on observations made by study co-author Daniela Rössler in 2020. While keeping local jumping spiders in transparent containers, she noticed them hanging motionless from silk lines. A 2022 paper suggested that Evarcha arcuata displayed a state similar to REM sleep, but the researchers had not yet established that the spiders were sleeping.
The authors say that structured sleep in spiders could help scientists investigate how sleep evolved across animals. Spiders and vertebrates, including mammals, belong to lineages that diverged roughly 500 million years ago. The presence of a REM-like phase in animals with very different brains may indicate that some basic features of sleep are ancient and widely distributed.
The study does not show whether spiders dream. Scientists have limited knowledge even about the relationship between REM sleep and dreaming in humans, making the question particularly difficult to answer in spiders. The researchers plan to compare sleep across different spider species and examine how laboratory conditions, artificial light and habitat disturbance affect spiders’ sleep in the wild.
Sources: Smithsonian Magazine.