Spiders May Have a Sleep Stage Like the One Where Humans Dream
Researchers at the University of Bonn watched an Australian jumping spider over several days and nights and found two sleep stages, one with eye movements and twitches like human dreaming sleep.
Spiders may have a sleep stage like the one in which humans dream. Researchers at the University of Bonn spent several days and nights watching an Australian jumping spider and found two clearly different stages of sleep, one of them marked by eye movements and muscle twitches — the kinds of movements people make during dreaming sleep.
A hunter about the size of a coin
The spider in the study was Portia fimbriata, a small Australian jumping spider that grows to about the size of a one-euro coin. It is an unusual hunter: instead of catching insects, it specialises in hunting other spiders. It does this with tricks, including copying the vibrations made by an insect struggling in a web. A web-building spider feels those vibrations, comes looking for what seems to be an easy meal, and finds the hunting Portia instead.
That kind of clever behaviour is why the researchers chose this species. They wanted to look at the link between sleep, the nervous system and the ability to learn. What they found were two sleep stages that could be told apart. One of them looks like the stage humans call REM, short for rapid eye movement. During that stage the spiders moved their eyes and twitched their muscles.
How do you tell a sleeping spider from a still one?
A spider sitting motionless is not necessarily asleep, so the team had to test it. Each spider was placed alone in a plastic dish 12 centimetres across and watched under different lighting conditions. Infrared cameras let the researchers film through the night without using visible light, which would have disturbed the animals. The recordings captured the spiders' sleeping positions, their quiet rest and their bursts of REM-like movement. One position was often seen: hanging upside down beneath a web the spider had spun itself.
The team also used sounds to see how quickly a spider reacted to a disturbance. First they had to find a sound that produced a steady, repeatable response — not a simple task, because spiders sense the world differently from people. The researchers then interrupted the spiders' rest to see whether they slept more afterwards, a pattern known as sleep rebound. Taken together, the recordings and the tests gave strong evidence that these spiders have genuine, regulated sleep with distinct stages, rather than simply going quiet.
What the study cannot say
The researchers could not measure the spiders' brain activity. They worked entirely from behaviour, which meant collecting high-resolution video over many days and nights and analysing a very large amount of material. The team is clear that seeing REM-like movements does not prove that spiders dream. Nobody has shown what, if anything, is going on in a spider's mind during that stage.
Sleep has been studied closely in people and in other animals with backbones, but far less is known about animals whose nervous systems are built very differently. Even so, this work is described as the first comprehensive experimental confirmation that spiders experience real, regulated sleep rather than plain inactivity. It also raises a bigger question: how did sleep evolve, and could animals with nervous systems very different from ours share some of its basic features?