The short wavelength, i.e. blue light, is crucial for non-image forming effects such as entrainment of circadian system. Moreover, many studies showed that blue light enhances alertness and performance in cognitive tasks. However, most of scientific reports in this topic is based on studies using short exposure to blue or blue-enriched light and only few focused on the effects of its reduced transmittance, especially in longer period. The latter could potentially give insight into understanding if age-related sleep problems and cognitive decline are related to less amount of blue light reaching the retina, as our lenses become more yellow with age. In this study, we investigated the effects of prolonged blocking of blue light on cognitive functioning, in particular – sustained attention and visuospatial working memory, and sleep. We used amber contact lenses reducing transmittance of blue light by approximately 90% for the period of four weeks on a group of young, healthy participants. No changes were observed for measurements related to sleep and sleep-wake rhythm. The significant effect was shown both for sustained attention and visuospatial memory, i.e. the longer blocking the blue light lasted, the greater decrease in performance was observed. Additionally, the follow-up session was conducted (approximately one week after taking off the blue-blocking lenses) and revealed that in case of sustained attention this detrimental effect of blocking BL is fully reversible. Our findings provide evidence that prolonged reduction of BL exposure directly affects human cognitive functioning regardless of sleep-related conditions.