Learning and memory are influenced by day and night, but the nature and mechanism of this modulation are still poorly understood. According to a new study conducted at the University of Houston, melatonin, a hormone released in a circadian way (ie following the phases of day and night), appears to be involved in these activities. The researchers formulated this hypothesis by studying the storage capacity of a diurnal fish, the zebrafish (danio rerio), whose DNA lends itself to comparative studies: the little fish had already been the protagonist, in 2005, of a research that led to identifying the gene that gives color to the skin. The new study, conducted through a series of conditions, showed that trained fish by day learn better and faster than those conditioned during the night, and the memory of the event lasts longer. The goal of the work, still far from being concluded, is to achieve an improvement in the mental performance of human beings.