Circadian Rhythm Adaptation Mechanisms in Extreme Environments

Abstract

Circadian rhythms coordinate physiology and behavior across species, yet on extreme photoperiodic cycles, in polar regions, in spaceflight, and deep sea, and in thermally extreme habitats, photic entrainment is disturbed. This review attempts to draw together evidence for clocks functioning well when light cues are unreliable; in doing this, it emphasizes non-photic Zeitgebers and molecular plasticity. The studies cite temperature cycles, altered gravity, feeding and social schedules, and magnetic inputs in terms of their abilities to entrain central and peripheral oscillators. We highlight at the molecular level epigenetic remodeling (DNA methylation, histone marks, and chromatin accessibility), non-coding RNAs, and posttranslational modifications as the key mechanisms that under stress retune the CLOCK: BMAL1–PER/CRY feedback loops in metabolism and immunity. A comparative appraisal includes polar vertebrates maintaining rhythms in continuous light/darkness via thermal and seasonal signals; astronauts and model organisms adapting in microgravity with desynchronized tissue clocks and altered neurocardiac dynamics; and desert, polar marine, and deep-sea species responding to temperature, tidal, and lunar cycles. Overall, these findings reveal the flexible multi-cue architecture for temporal organization that directly pertains to human health and performance concerns in polar operations and long-duration spaceflight, as well as ecological resilience in rapid environmental change. The journey into how clocks reweight non-photic inputs and engage epigenetic flexibility underpins countermeasures (light, temperature, feeding schedules), which could lead to conservation methods aimed at reestablishing rhythmicity during conditions when photic cues become dysfunctional.