Spiders Pay Back the Dream-Like Sleep They Lost
A jumping spider that misses a night of REM-like sleep spends the next night making it up.
A University of Bonn team has a result that does not need the word dream to be strange. In PNAS this week they report that the fringed jumping spider, Portia fimbriata, keeps two regulated sleep stages. One of them looks like REM: the eyes move, the body twitches, and the animal is hard to treat as merely "resting."
The test was rude and specific. Researchers kept spiders awake at night by putting 25 live fruit flies in the dish. The next night, those spiders spent nearly 80 percent more time in the REM-like state. The extra time arrived as more episodes, not as longer ones. That is the shape of a debt being paid, not of an animal that simply slept in.
The Bonn group says this is the first demonstration of REM rebound in an invertebrate. Vertebrates have been doing this trick under a microscope for decades. A spider that hangs upside down under a web it spun for the purpose, then makes up a lost night, is a different sentence.
Key Evidence
- University of Bonn news release, 27 October 2026, on a PNAS paper (doi:10.1073/pnas.2618030123)
- Species: Portia fimbriata, an Australian jumping spider
- Two regulated sleep stages, one with eye movements and muscle twitches
- Sleep disruption: 25 live fruit flies placed in the dish overnight
- Next night: nearly 80 percent more time in REM-like behaviour, as more episodes rather than longer ones
- Authors' claim: first REM rebound shown in an invertebrate
- Secondary walkthrough: ScienceAlert, same week
The Rational Explanation
Eye twitches and a recovery night show a regulated state. They do not show pictures in a head. "Dream" in the headlines is a metaphor the paper does not require, and a careful desk should not smuggle it back in. A spider's nervous system can keep a homeostatic counter without hosting a narrative. The flies may have stressed the animal in ways that are not "lost sleep" in the human sense. Replication in another lab, and in a less theatrical spider, would tighten the claim.
What We Don't Know
Nobody can ask the spider what, if anything, it experienced. The paper does not show imagery. It shows a rebound. Whether that rebound is doing the same job as human REM — memory, temperature, a developmental leftover — is open. So is the question of how far down the tree the debt goes. One genus is not the invertebrates.
The Rabbit Hole
Sleep used to be the thing we granted to animals with faces. Then it turned up in jellyfish, in fruit flies, and in a stack of papers that made "unconscious" look like a sloppy word. REM was the holdout: the stage people point at when they want to say dreaming is special. If a spider pays that stage back, the specialness moves. It does not vanish. It just stops being a vertebrate patent.