Sleep is regulated by a circadian clock and by a homeostatic process whose behavioral signature is compensatory rebound sleep after deprivation; because rebound appears nearly universal, homeostatic regulation is widely treated as the defining, conserved property of sleep. A comparative study of seven Drosophila species (Joyce et al., Nature Communications 15:5091, 2024) found that circadian aspects of sleep are conserved while homeostatic regulation is not: three species rebound only after social deprivation and three show no rebound at all, yet all conserve two stages of sleep depth. The authors proposed that sleep in Drosophilids evolved primarily to serve circadian-driven adaptive inactivity, with homeostatically regulated functions added later, independently and species-specifically. The open problem is to settle this divergent-evolution model with causal evidence: whether homologous synaptic-plasticity pathways are less active, differently regulated, or functionally diverged in non-rebounding species; whether mechanical and social rebound are distinct processes; what deep sleep does without homeostatic control; which molecular changes underlie the diverged dopaminergic pharmacology; which ecological pressures drove the divergence; and whether circadian primacy generalizes beyond Drosophilids. No study has yet resolved any of these threads; the problem is open.

Background

In animals, sleep is governed by two interacting processes. The first is the circadian clock, the evolutionarily conserved molecular oscillator that synchronizes physiology and behavior to the 24-hour day. The second is homeostatic regulation: the pressure to sleep accumulates during wakefulness and is discharged by sleeping. Its clearest behavioral readout is rebound - the compensatory increase in sleep amount and depth observed after experimental sleep deprivation. Because nearly every animal studied shows rebound, homeostasis is widely regarded as a defining and conserved feature of sleep, and hence as the strongest evidence that sleep serves some universal biological function beyond keeping animals safely inactive. The fruit fly Drosophila melanogaster has been a leading genetic model for sleep since sleep-like states were first described in the fly around 2000 (reviewed by Shafer, Fly 19:2502180, 2025). Fly sleep is defined behaviorally - immobility lasting at least five minutes (the conventional "5-minute rule") with reduced responsiveness to stimuli - but it is not a unitary state: arousal-threshold measurements and behavioral staging distinguish lighter from deeper sleep bouts. A major line of fly work links sleep pressure to synaptic strength: prolonged wakefulness increases brain levels of the presynaptic scaffold protein Bruchpilot (BRP), and sleep reverses this increase, supporting the idea that sleep homeostatically regulates synaptic plasticity. RNA interference (RNAi), the knock-down of gene expression by double-stranded RNA, has been used with neuron-specific drivers to tie this plasticity mechanism to specific brain regions. In D. melanogaster, pan-neuronal knock-down of four genes involved in synaptic plasticity - dunce (a cyclic-AMP phosphodiesterase), synapsin (a regulator of synaptic vesicles), starry night (a regulator of synaptic morphogenesis) and appl (the fly analogue of the vertebrate amyloid precursor protein) - selectively reduces or eliminates rebound after mechanical sleep deprivation while sparing significant rebound after social deprivation, reproducing the profile of the species that lack mechanical rebound.

Problem Statement

Following Joyce et al. (2024), the problem is to explain, with causal evidence, why homeostatic sleep regulation diverged across Drosophila species, and thereby to adjudicate their model that sleep in this group evolved primarily to serve circadian-driven adaptive inactivity (keeping animals immobile during the dangerous hours of the day) while homeostatically regulated sleep functions were added independently, species by species. Six linked questions identify the open directions of the model. The causal core of the model is the mechanistic basis of absent rebound (1) and the relation between the two rebound processes (2); questions (3)-(6) are further directions of the model. (1) Mechanistic basis of absent rebound: is the absence of rebound after mechanical sleep deprivation in D. erecta, D. willistoni and D. virilis, and the social-only rebound of D. simulans, D. sechellia and D. yakuba, caused by homologous synaptic-plasticity pathways (such as the dunce, synapsin, starry night and appl pathways defined in D. melanogaster) that are less active, differently regulated, or functionally diverged in those species? (2) Two rebound processes: are the rebounds after mechanical deprivation and after social (forced male-male interaction) deprivation different manifestations of one homeostatic mechanism, different triggers of overlapping mechanisms, or distinct circuits serving different biological functions (synaptic regulation versus stress recovery)? (3) Deep sleep without homeostasis: what function do the conserved stages of deep sleep - present in species that never rebound - actually serve; in particular, do they perform a non-homeostatic function such as waste clearance or energy conservation that predates the evolution of homeostatic control? (4) Diverged neuropharmacology: which molecular differences in dopamine receptor expression, downstream signaling or metabolism explain that the dopamine precursor L-dopa is wake-promoting only in D. melanogaster while it is sleep-inducing (somniferous) in D. sechellia and D. willistoni? (5) Ecological drivers: which ecological pressures or niche features explain why some species evolved strong homeostatic rebound and others none? (6) Generality: does the circadian-primacy, divergent-homeostasis model hold in other insect orders, other arthropods, or vertebrate taxa? The model is scoped to Drosophilids, and wider generality is raised only as a conjecture ("This may be the common process of sleep evolution in the animal kingdom"); threads (5)-(6) concern the ecological logic of the divergence and whether the model extends beyond the group. Refuting the premise - for example, evidence that rebound in supposedly non-rebounding species was obscured by the scoring convention - remains a possible empirical outcome of these questions.

References

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