Circadian Rhythm Mechanisms Underlying Convergent Adaptation of Unihemispheric Slow-Wave Sleep in Marine Mammals

Marine mammals have evolved unihemispheric slow-wave sleep, a unique state during which one cerebral hemisphere sleeps while the other remains awake, to mitigate the fundamental conflict between sleep and wakefulness. However, the underlying mechanisms remain largely unclear. Here, we use a comparative phylogenetic approach to analyze genes associated with light-dependent circadian mechanisms, aiming to reconstruct the evolution of the circadian rhythm pathway in mammals and to identify adaptively changed components likely to have contributed to the development of unihemispheric slow-wave sleep. Specifically, among eight genes with shared signals of positive selection in two unihemispheric slow-wave sleep-specific lineages, seven genes showed direct evidence of affecting sleep and spontaneous movements. Both in vitro and in vivo experiments indicated that functional innovation in cetacean and non-phocid pinniped FBXL21, which was found to undergo positive selection, may be beneficial for decoupling sleep-wake patterns from daily rhythms to sustain continuous swimming. For cetaceans exhibiting only unihemispheric slow-wave sleep, we identified 73 genes as rapidly evolving and 92 genes containing unique amino acid substitutions. Functional assays showed that a cetacean-specific mutation (F411Y) in NFIL3 led to a decrease in repressor activity and protein stability. Furthermore, convergent amino acid replacements detected in genes related to Ca2+ signaling and CREB phosphorylation suggest their crucial role in unihemispheric slow-wave sleep adaptation. Overall, this study enhances our understanding of the evolutionary mechanisms underlying unihemispheric slow-wave sleep and provides a foundation for investigating how circadian rhythm changes contribute to variations in sleep and circadian behavior.