The question of whether an organism as seemingly simple as a spider truly sleeps has long puzzled biologists. Sleep is a universal biological requirement across the animal kingdom, but its definition often relies on complex brain activity that many invertebrates lack. For decades, the resting periods of spiders were considered mere inactivity or torpor. However, recent scientific observation has revealed behavior in these eight-legged creatures that suggests a more complex, regulated state of rest akin to slumber in vertebrates. This finding forces a re-evaluation of sleep’s evolutionary origins, pushing the timeline far back before the emergence of mammals and birds.
Defining Sleep in the Animal Kingdom
Scientists define true sleep using a set of observable behavioral and physiological criteria. The primary behavioral characteristic is a period of sustained immobility or behavioral quiescence, often associated with a specific resting posture. This state must also be accompanied by a significantly reduced responsiveness to external stimuli, meaning the animal is harder to rouse than if it were merely resting. Importantly, this reduced state must be rapidly reversible; the animal can quickly return to full alertness when presented with a strong enough stimulus, which distinguishes sleep from states like hibernation or coma.
Beyond behavior, sleep is also defined by homeostatic regulation, a process where lost sleep is followed by a compensatory period of increased rest, often called “sleep rebound.” These criteria allow researchers to identify sleep in animals that do not possess the complex brain structures necessary for electroencephalogram (EEG) measurements. While mammals and birds exhibit distinct phases like Rapid Eye Movement (REM) and non-REM sleep, the fundamental behavioral hallmarks of immobility, reduced awareness, and rapid reversibility remain the standards for defining a sleep state across species.
Observing Spider Rest Behavior
Before recent discoveries, the periods of inactivity observed in spiders were largely classified as simple rest or energy-conserving torpor. Many species of spiders, particularly the web-building types, remain motionless for extended periods while waiting for prey. During these hours, the spider is poised and vigilant, ready to spring into action at the first vibration, which is a state fundamentally different from true sleep.
When truly resting, spiders often adopt a characteristic posture that differs noticeably from their alert, hunting stance. Species like the European jumping spider, Evarcha arcuata, were observed suspending themselves upside down from a single silk thread to rest at night. Their legs are often pulled in close to the body, a position that minimizes the energy required for maintenance.
The Discovery of REM-like States
The understanding of spider rest shifted dramatically with the close observation of juvenile jumping spiders. Researchers used infrared video to record the nocturnal resting behavior of the young Evarcha arcuata, whose exoskeletons are temporarily translucent. This transparency allowed scientists to directly view the internal structures of the spider’s eyes, specifically the movable retinal tubes. Jumping spiders are highly visual hunters and use these tubes to redirect their gaze, providing a unique window into their nervous system activity.
During periods of deep rest, the spiders exhibited regularly recurring bouts of specific motor activity. These bouts included uncontrollable twitching of the legs and a stereotypic curling of the limbs, which is thought to result from the relaxation of muscles in the head that regulate hydraulic pressure in the legs. The most compelling evidence emerged when the twitching coincided precisely with rapid, ballistic movements of the retinal tubes.
These periods of rapid retinal movement and limb twitching occurred in cycles, with consistent durations and intervals, increasing in length over the course of the night. The combination of periodic limb movements and rapid eye activity strongly paralleled the characteristics of Rapid Eye Movement (REM) sleep seen in vertebrates. The average duration of these REM sleep-like bouts in the spiders was similar to those observed in small mammals.
This groundbreaking discovery marks the first observation of an REM sleep-like state in a terrestrial invertebrate, suggesting that this sophisticated sleep phase is evolutionarily far older than previously thought. Although brain activity cannot be measured in these tiny arachnids, the behavioral and ocular evidence strongly indicates a regulated sleep phase beyond simple rest. The finding opens the door to new research on the function of sleep in organisms with vastly different neurological structures.