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Artificial light at night leads to circadian disruption in a songbird: integrated evidence from behavioural, genomic and metabolomic data

Globally increasing levels of artificial light at night (ALAN) are associated with shifts in circadian rhythms of behaviour in many wild species. However, it is still unclear whether changes in behavioural timing are underlined by parallel shifts in the molecular clock, and whether such internal shifts may differ between different tissues and physiological pathways, which could highlight circadian disruption. We tackled these questions in a comprehensive study that integrated behavioural, gene expression and metabolomic analyses. We exposed captive male great tits (Parus major) to three ALAN intensities or to dark nights, recorded their activity rhythms and obtained mid-day and midnight samples of brain, liver, spleen and blood. ALAN advanced wake-up time, and this shift was paralleled by an advance in the expression of the clock gene BMAL1 in all tissues, suggesting close links of brain and peripheral clock gene expression with activity rhythms. However, several metabolic and immune genes were desynchronised the shifted BMAL1 expression, suggesting circadian disruption of behaviour and physiology. This result was reinforced by untargeted metabolomic profiling, which showed that only 9.7% of the 755 analysed metabolites followed the behavioural shift. We suggest circadian as a key mediator of the health impacts of ALAN on wild animals.