Adaptive changes in BMAL2 with increased locomotion associated with the evolution of unihemispheric slow-wave sleep in mammals

From Wings to Wellness: A Research Agenda Inspired by Migratory Bird Adaptations for Sleep and Circadian Medicine

Migratory birds demonstrate remarkable temporal plasticity, adapting their circadian rhythms and sleep patterns to meet the demands of long-distance migration. This perspective explores how insights from avian temporal adaptations could inform novel research directions in human sleep and circadian medicine. Birds' ability to maintain precise temporal organization through multiple coordinated oscillators, particularly during migratory periods, provides a valuable framework for understanding circadian flexibility. Drawing from recent advances in avian chronobiology, we propose several research priorities for human applications, including biomarker-guided chronotherapy, circuit-specific interventions, and optimization of environmental cue timing. We explore how birds' sophisticated control of sleep architecture and metabolic regulation during migration might inspire new approaches to managing circadian disruptions in humans. Neuroimaging studies of human temporal adaptability, guided by avian insights, could reveal network-level mechanisms underlying circadian plasticity. Of particular interest is the parallel between avian unihemispheric sleep and human hemispheric asymmetry during sleep, suggesting the evolutionary conservation of adaptive sleep mechanisms. While acknowledging the fundamental differences between avian and human circadian systems, we outline specific research directions that could translate avian temporal adaptability principles into therapeutic strategies for circadian disorders. While these avian-inspired hypotheses require rigorous validation, and some may not prove viable, embracing creative exploration remains essential for advancing our understanding of human circadian biology and guiding the development of novel therapeutic approaches.

Evolution of canonical circadian clock genes underlies unique sleep strategies of marine mammals for secondary aquatic adaptation

To satisfy the needs of sleeping underwater, marine mammals, including cetaceans, sirenians, and pinnipeds, have evolved an unusual form of sleep, known as unihemispheric slow-wave sleep (USWS), in which one brain hemisphere is asleep while the other is awake. All aquatic cetaceans have only evolved USWS without rapid eye movement (REM) sleep, whereas aquatic sirenians and amphibious pinnipeds display both bihemispheric slow-wave sleep (BSWS) and USWS, as well as REM sleep. However, the molecular genetic changes underlying USWS remain unknown. The present study investigated the evolution of eight canonical circadian genes and found that positive selection occurred mainly within cetacean lineages. Furthermore, convergent evolution was observed in lineages with USWS at three circadian clock genes. Remarkably, in vitro assays showed that cetacean-specific mutations increased the nuclear localization of zebrafish clocka, and enhanced the transcriptional activation activity of Clocka and Bmal1a. In vivo, transcriptome analysis showed that the overexpression of the cetacean-specific mutant clocka (clocka-mut) caused the upregulation of the wakefulness-promoting glutamatergic genes and the differential expression of multiple genes associated with sleep regulation. In contrast, the GABAergic and cholinergic pathways, which play important roles in promoting sleep, were downregulated in the bmal1a-mut-overexpressing zebrafish. Concordantly, sleep time of zebrafish overexpressing clocka-mut and bmal1a-mut were significantly less than the zebrafish overexpressing the wild-type genes, respectively. These findings support our hypothesis that canonical circadian clock genes may have evolved adaptively to enhance circadian regulation ability relating to sleep in cetaceans and, in turn, contribute to the formation of USWS.