ABSTRACT Environmental light, particularly during early development, significantly influences lateralisation, the asymmetric information processing between brain hemispheres. We hypothesised that lateralisation could be affected by artificial light at night (ALAN), a widespread form of environmental pollution. In our experiment, we exposed eggs and larvae of zebrafish to either control or ALAN conditions and then tested them in a rotational test to assess motor lateralisation, and a mirror test to assess lateralisation in response to visual stimuli. The control group exhibited a significant lateralisation bias at the population level, prioritising the processing of visual information with their right hemisphere. In contrast, the zebrafish exposed to ALAN did not show this bias, leading to a notable reduction in lateralisation. Additionally, we found evidence of reduced individual differences in lateralisation in the ALAN group. Overall, our findings demonstrate that ALAN disrupts the natural lateralisation in fish larvae, possibly affecting their behaviour and survival.