Tiny Jumping Spiders Twitch at Night and Can Build Up Sleep Debt

A jumping spider a few millimetres across settles down for the night. Then its legs begin to jerk, its mouthparts tremble, and the retinas inside its eyes flick back and forth, as though it were following something that is not there. Researchers say this is the best evidence yet that spiders go through a stage of sleep like the rapid-eye-movement sleep of people, the phase when human eyes dart about and dreams are most often reported. And when the spiders were kept from resting, they later appeared to make up the lost rest, much as a tired person does.

What the sleeping spider actually does

The spider is Portia fimbriata, the fringed jumping spider, and an adult male is only a few millimetres across. Filmed with infrared cameras, which let the researchers watch in darkness, he was not still for long. Periods of stillness were regularly broken by bouts of movement: twitching limbs, trembling mouthparts, and retinas that swung from side to side. The retinas are the light-sensitive parts of the eye, and they were the most striking detail, because they seemed to be tracking motion that was not there. The researchers also recorded movement in the abdomen and in the spinnerets, the small organs spiders use to spin silk.

How they showed the spider was asleep, not just resting

Twitching on its own proves nothing. Sleep has to be demonstrated. An animal that is truly asleep holds a typical posture, responds less to what is going on around it, can still be woken by a strong enough disturbance, and, crucially, pays back rest it has missed. Portia was mostly active during the day and settled into a resting position at night. It became less responsive to disturbances while resting, especially during the twitchy episodes. Then the team disrupted the spiders' nightly rest to see what happened afterwards. The researchers point to what they call the homeostatic control of sleep, the tendency of a sleeper to catch up on rest that has been lost. That catching-up is what sleep debt means.

An earlier clue, and a spider with a reputation

In 2022, a team led by Rößler reported a similar state in another jumping spider, Evarcha arcuata. Those spiders also twitched, and their retinas tracked movements that were not there. But the researchers had not yet established that the animals were actually asleep, and a sleep stage cannot be confirmed without the sleep. Portia was a good next subject because of its mind. It can plot a complicated route to prey after losing sight of the target, and it can trick other spiders into coming closer by plucking their webs to imitate the vibrations of a trapped insect. When one tactic fails, it tries another. Geiger and Rößler, behavioural ecologists at the University of Bonn in Germany, told ScienceAlert that what surprised them was how closely spider sleep mirrors the sleep of animals with backbones in its structure and regulation. They said the findings support the idea that distinct sleep stages, including active sleep, may have very deep evolutionary roots, and noted that comparable stages have now been described in animals as distant from one another as spiders, octopuses and squid, and animals with backbones. The work was published in the journal PNAS.

Whether a spider dreams is a different question

Nobody can ask Portia what it sees. What the researchers can say is that the movements look like the ones a dreaming person makes, and that a spider's night now looks less strange than it did. Sleep itself remains one of biology's biggest mysteries: scientists know a great deal about what happens during it, but not why it evolved in the first place.