The cyclical changes of day and night cause animals to exhibit rhythms in behaviour (specific sleep and activity times), physiological processes (blood pressure, blood hormone levels, etc.), as well as diurnal changes at the cellular level (changes in communication between cells, changes in cell shape) and molecular level (production levels of specific proteins). All these rhythms in living organisms are regulated by biological clocks, which are located in the brain and numerous peripheral tissues. Such oscillators can be synchronised by external conditions (light intensity, temperature, food availability), but can persist for long periods of time under constant conditions, e.g. in total darkness. Oscillator cells are characterised by the expression of several clock genes, the levels of which change cyclically throughout the day. The endogenous clock mechanism is controlled by light through the cryptochrome (CRY) protein, which is a blue light photoreceptor. Dr Milena Damulewicz from the Department of Cell Biology and Imaging at the Jagiellonian University is conducting research on the regulation of circadian rhythms in the visual system. The best model for such research is the fruit fly, drosophila melanogaster, in which the clock mechanism is well understood and at the same time similar to that described in mammals, so that the results obtained in insect studies allow us to better understand the mechanisms of regulation of circadian processes in humans. In a long-term collaboration with Prof Gabriella Mazzotta from the University of Padua, new functions of CRY in the regulation of synaptic plasticity as well as behavioural processes such as sleep have been described. Among other things, it was shown that in the visual system, the CRY protein can form complexes with the presynaptic protein BRP, so that it is degraded. This process occurs during the day when CRY is activated by light. Changes in BRP protein levels during the day affect the number and size of synapses at the retinal photoreceptor terminals, which in turn allows for precise regulation of the level of perception and transmission of visual stimuli to deeper parts of the brain. As a result, by regulating the activity of clock neurons throughout the day, the CRY protein influences the activity and sleep levels of insects. This result shows how important a role light plays in regulating physiological and behavioural processes. The knowledge gained about the influence of light on brain function is very important, as it suggests that excessive exposure to light may cause not only sleep disorders, but also normal visual processes. Polish-Italian cooperation in this research was made possible thanks to grants from the National Science Centre and the Bekker NAWA fellowship.