RIS and ALA orchestrate sleep in trigger-specific ways. Both are engaged through distinct, largely conserved molecular signaling mechanisms. Thus, sleep research in C. elegans allows insights into molecular pathways that contribute to the regulation of sleep across species, including humans. Examples and potential translational applications are discussed below.

AP2 transcription factors control sleep across species

In humans, heterozygous loss of AP2 transcription factor function causes Char syndrome, which is linked to self-reported sleep disturbances [24]. However, no causal link between AP2 transcription factor activity and sleep has been established in humans to this day, and the underlying mechanisms are not understood. In C. elegans, however, causality and the molecular mechanism have been elucidated: the AP2 transcription factor APTF‑1 is essential for expressing the sleep-promoting peptide FLP-11 in RIS [33]. A genetic knockout of aptf‑1 completely abolishes most types of sleep. These findings prompted efforts to translate this knowledge into animals with larger brains. Indeed, AP2 transcription factors were subsequently shown to regulate sleep in both fruit flies and mice [13, 19].

Together, these findings show that AP2-dependent sleep control is conserved across large evolutionary distances from worms to rodents, suggesting that it likely extends to humans as well. If so, the mechanistic insights gained from C. elegans should contribute to understanding and ultimately treating human sleep disturbances like those linked to Char syndrome. More broadly, the progression from C. elegans to mice demonstrates that worm sleep and mammalian sleep are, at least in part, controlled by homologous genetic regulators, underscoring the translational relevance of insights from simple model organisms.

EGF receptor signaling and salt-inducible kinase 3 couple cellular stress, metabolism, and sleep

Epidermal growth factor receptor (EGFR) signaling has emerged as a conserved regulator of sleep, particularly in the context of illness and physiological stress. A role for EGFR in sleep regulation and locomotion activity has been demonstrated in mouse studies [18]. In humans, common variants in genes within the EGFR signaling pathway are associated with variation in sleep quantity and quality [21]. However, the precise molecular mechanisms of how EGFR controls sleep in mammals remain unresolved.

In C. elegans, a molecular pathway involving EGFR has been described by which damaged peripheral tissue signals to the nervous system to induce sleep: following cellular stress such as heat shock, the C. elegans EGF ligand SISS‑1 is shed from the affected tissues and signals to the EGFR called LET-23 that is expressed in ALA and RIS. This pathway induces protective sleep that enhances survival [11, 16]. Although its potential for clinical applications remains speculative at present, the evolutionary conservation of EGFR-induced sleep highlights this pathway as a fundamental mechanism by which animals couple peripheral stress detection to increased protective sleep.

Moreover, research in C. elegans suggests that EGF signaling may integrate not only stress signals but also metabolic cues into sleep control. In 2008, the C. elegans homolog of the salt-inducible kinase SIK3, KIN-29, was analyzed for its role in chemosensory signal-dependent gene expression and behavior. At that time, worms were known to display quiescence behavior after feeding on high-quality food, which was compared to mammalian satiety behavior but not yet referred to as sleep. KIN-29 mutants showed defective food-induced quiescence [36]. A functional link between SIK3 and sleep, however, was established in 2016, when SIK3 was found to control sleep homeostasis in randomly mutated mice [5]. Later, work in C. elegans elucidated a molecular mechanism in which the SIK3 homolog KIN-29 acts upstream of EGF to integrate the metabolic state and fat storage into sleep regulation [7]. The work suggests that chronic obesity might promote short sleep through SIK3-dependent mechanisms. This idea is consistent with the observation that short sleep and elevated fat stores in humans are associated.

AMP-activated kinase and insulin signaling link protective gene expression and sleep

The nutrient-sensing adenosine monophosphate-activated protein kinase (AMPK) and insulin signaling pathways present additional conserved molecular links between metabolism and sleep. C. elegans larvae that hatch in the absence of food enter a developmental stage called L1 arrest. In this condition, they show starvation-induced sleep, which is regulated by AMPK and insulin signaling [37]. This pathway activates RIS to induce sleep and drives protective gene expression through activation of the worm homolog of FOXO, called DAF-16, a pro-longevity transcription factor inhibited by insulin signaling [17]. The conservation of these mechanisms is underscored by findings in other species: FOXO regulates starvation-induced sleep in the fruit fly, whereas AMPK contributes to sleep homeostasis in mice. From a medical perspective, this raises intriguing possibilities. If AMPK- or FOXO-dependent pathways can be modulated to activate protective programs including sleep, they may offer new strategies for mitigating stress-related pathologies and insomnia, for example via lifestyle interventions.

Antimicrobial peptides signal sleep need as part of an innate immune response

Antimicrobial peptides and cytokines are traditionally viewed as immune effectors but have long been suspected to influence sleep regulation during infection, wounding, and inflammation. Evidence for their involvement has been established via experimental ectopic expression or injection of antimicrobial peptides directly into the brain, which has been shown to increase sleep in mammals and the fruit fly [14, 32]. However, knockouts of individual cytokines or single antimicrobial peptides in the fruit fly had only minimal effects on behavior, prohibiting final mechanistic conclusions.

This lack of a strong sleep phenotype for single antimicrobial peptide-knockouts was also true for research in C. elegans. However, the advantages of this model allowed researchers to go a step further and generate a mutant strain that lacked not only individual antimicrobial peptide-genes but 19 of these genes simultaneously. These multi-knockout animals showed significantly reduced sleep after injury. This effect was specific, as sleep timed by the genetic oscillator remained unchanged in these animals. This enabled the demonstration of a highly redundant function for antimicrobial peptides as somnogens, underlining their importance in mediating sleep as part of the immune response [29]. Moreover, for one antimicrobial peptide, a receptor and target neurons have been identified, presenting the first demonstration of an actual long-range, cross-tissue signaling function for antimicrobial peptides in sleep regulation. Whether human antimicrobial peptides work through a similarly redundant mechanism is an interesting question for future research. Given the growing interest in peptide-based therapeutics, antimicrobial peptides could become promising candidates for treating immune-related sleep disturbances or postoperative fatigue [38].