Jumping spiders sleep in two stages, including a REM-like phase, a Bonn study finds

Jumping spiders sleep, and their sleep has two stages, one of which resembles the REM sleep of humans, according to a study published in PNAS on 27 October, the University of Bonn said. The researchers describe it as the first comprehensive confirmation of sleep behavior in a spider.

Testing whether a spider is really asleep

The team studied Portia fimbriata, an Australian jumping spider about the size of a one-euro coin when fully grown. Spiders were kept in small plastic dishes, 12 cm across, and filmed over several days and nights under changing light, with infrared cameras at night. The researchers recorded sleep postures, resting phases and REM-like phases, in which the spiders made eye movements and muscle twitches.

Resting alone is not sleep, so the team ran two classic tests. They played a sound and measured how quickly the spiders reacted, since sleeping animals respond more slowly to the outside world. And they disturbed the spiders' sleep to see whether they made up for it later, a response called sleep rebound. Together, these showed regulated sleep in two stages, the study found.

Eyes that move inside the head

The work builds on a 2022 study, also in PNAS, led by Daniela Rößler, who now heads the Bonn group. Jumping spiders cannot move the lenses of their eyes but can move their retinas to shift their gaze. In young spiders of another species, whose bodies are still see-through, her team filmed regular bouts of retinal movement at night, paired with leg curling and twitches, and proposed a REM sleep-like state. Until then, REM-like sleep had been described only in animals with backbones, the 2022 paper noted.

The new study goes further by showing that the resting state meets the behavioral tests for sleep and that the REM-like phase is part of a regulated pattern. The research was funded by the German Research Foundation through its Emmy Noether Program, and co-author Nadja Geiger is also at the Bonn Institute for Organismic Biology.

Behavior only, so far

The team recorded behavior only and did not measure brain activity, so it cannot say what happens in the spiders' nervous system during either stage. Finding a stimulus that reliably woke the spiders was difficult, because spiders perceive the world very differently from people, and the video data took a long time to analyze.

Next, the researchers want to trace how REM-like sleep evolved across spiders, compare wild and laboratory animals, look for links between sleep and the spiders' problem-solving abilities, record brain activity in arachnids and insects, and test how artificial light at night affects their sleep. Studying sleep in animals with very different nervous systems could help reveal what sleep was originally for, the authors say.