The question of whether spiders experience anything akin to sleep has long fascinated observers. While their eyes don’t close like eyelids, these arachnids exhibit profound stillness, prompting curiosity about their resting behaviors. Recent observations are unraveling the complexities of spider inactivity, challenging previous assumptions about rest.
Do Spiders Truly Sleep?
For a long time, it was assumed spiders did not genuinely sleep like mammals, attributing their inactivity to simple rest or energy conservation. However, recent scientific inquiry suggests some spiders enter a distinct sleep-like state of reduced activity, differing from mammalian rapid eye movement (REM) sleep. Like many animals, spiders possess circadian rhythms, internal biological clocks regulating their daily periods of activity and rest.
What Spider “Sleep” Looks Like
Spider “sleep” involves observable behaviors distinct from mere stillness. In jumping spiders like Evarcha arcuata, researchers observe a specific nocturnal resting posture: suspending themselves upside down from a single silk thread. During these periods, spiders exhibit periodic limb movements, including leg twitching, curling, and subtle movements of their opisthosoma and spinnerets.
A striking observation in juvenile jumping spiders is the rapid flickering of their retinal tubes, internal structures used to adjust their gaze. Young jumping spiders have temporarily translucent exoskeletons, making these internal eye movements directly visible. The combination of limb twitches and retinal tube movements has led scientists to describe this state as “REM sleep-like,” paralleling the active sleep phase in vertebrates, including humans, where dreaming often occurs. These active phases occur in regular cycles throughout the night.
How Scientists Study Spider Rest
Investigating sleep-like states in invertebrates like spiders presents unique challenges, as traditional methods such as electroencephalography (EEG) used for mammals are not applicable. Instead, scientists primarily rely on non-invasive behavioral analysis, often using video recording. This approach allows researchers to document subtle movements and postures associated with spider inactivity.
The transparency of newly emerged jumping spiderlings proves invaluable, offering a direct view of internal processes like retinal tube movements. Researchers observe behavioral cues such as reduced movement, specific body postures, and decreased responsiveness to external stimuli to identify resting states. For example, a resting spider might remain motionless for extended periods and be less reactive to vibrations or light changes. This detailed observational approach, particularly in jumping spiders, has been instrumental in uncovering their rest patterns and suggesting a more sophisticated sleep-like state than previously thought.
Why Spiders Need Inactivity
Spiders engage in periods of inactivity for several biological reasons, serving functions similar to sleep in other animals. A primary benefit is energy conservation. By lowering their metabolic rate during these quiescent periods, spiders conserve energy, especially important for ambush predators or those that may go long periods between meals. This reduced activity allows them to survive periods of food scarcity or unfavorable environmental conditions.
Furthermore, these inactive states may be important for processing sensory information and consolidating memories. Spiders, particularly jumping spiders, exhibit spatial learning and memory, which could be strengthened during rest. While direct evidence is still emerging, it is hypothesized these periods contribute to physiological restoration, similar to sleep’s restorative functions in more complex organisms. This includes preparation for significant physiological events, such as molting, during which spiders are particularly vulnerable and require stillness.