Impact of artificial light at night on zebrafish circadian rhythms: Insights from behavioural and molecular data

Artificial light at night (ALAN) is an emerging environmental stressor that alters natural light cycles and disrupts circadian regulation across a wide range of species and ecosystems. By interfering with the environmental cues that have shaped circadian timekeeping mechanisms, ALAN poses a growing concern for circadian physiology and behaviour. In this study, we investigated the effects of ALAN on behavioural and molecular rhythmicity in the diurnal teleost Danio rerio, the zebrafish. Fish were exposed to either a natural light-dark (L:D) cycle or a light-ALAN (L:ALAN) regimen. Locomotor activity recordings revealed alterations in behavioural pattern mostly associated to arrhythmicity under ALAN, whereas L:D exposed fish exhibited robust circadian rhythms with clear diurnal patterns. To explore the molecular basis of this disruption, we performed RNA sequencing on brain tissue collected at four time points across the day. Transcriptomic analysis revealed significant alterations in the expression of core circadian clock and opsin genes in ALAN exposed zebrafish. Both positive and negative regulators of the molecular clock exhibited reduced rhythmic amplitude, indicating a dampening of endogenous circadian oscillations. These results demonstrate that ALAN profoundly affects both behavioural and molecular circadian rhythms in zebrafish. Given the widespread use of artificial lighting and its known ecological impacts, our findings highlight the need for integrative approaches, linking molecular, physiological, and behavioural data, to better understand the biological impacts of light pollution. Such insights are critical for informing conservation strategies and mitigating the effects of ALAN on wildlife and ecosystems.