Introduction. Given the global trend of increasing life expectancy and the importance of preserving cognitive capacities during aging, there is a need to determine the importance of all potential factors that may influence the trend of cognitive decline during aging. The method of event-related potentials (ERP), especially P300 wave, have been used for a long time in assessing of individual’s cognitive state and it is believed that P300 wave latency indicates the speed of cognitive stimulus processing and its amplitude the degree of attention resources. Recently, the concept of cognitive reserve stands out, which refers to the greater adaptability of cognitive processes in people who are more educated, had more responsible occupations in life and had greater involvement in leisure activities. Greater efficiency of cognitive processes in these individuals may partly explain the existence of interindividual differences in cognitive decline during normal aging but also in the case of various pathological and traumatic brain conditions. Sleep in the elderly is of poorer quality and the most striking change in sleep architecture during aging is the gradual reduction of slow-wave sleep, which is of great importance for improving cortical plasticity and increasing the clearance of metabolic by-products. Insomnia and other sleep disorders have also been shown to have negative effects on hippocampal neurogenesis, prefrontal cortex functions and overall cognitive state, which creates the need for a detailed study of the interrelationships between aging, sleep, and cognitive function. Aim. The aim of this study was to determine the neurophysiological correlates of cognitive aging and to determine the possible association of cognitive status with cognitive reserve and sleep quality in the elderly. Material and methods. The study was designed as a cross-sectional study conducted on a total of 106 subjects of both genders, aged 65-80 years (72±4.46 years), without established cognitive and neuropsychiatric diseases. Using the standard auditory oddball paradigm, the parameters of event-related potentials were determined, ie the amplitude and latency of P300 waves on Fz and Cz electrode sites, the choice reaction time, the percentage of correct answers and the number of errors during testing. The cognitive status of the respondents was additionally assessed using the Montreal Cognitive Assessment (MoCA) and the cognitive reserve index using a special questionnaire. The Pittsburgh Sleep Quality Index was used to assess the quality of sleep of the respondents. Prior to conducting the survey on a sample of elderly people, a pilot study was conducted on 223 respondents aged 18-85 years (52.34±18.46 years), which aimed to assess the internal consistency of the questionnaire used to assess the index cognitive reserves (CRIq) in this study after which its good reliability was determined (Cronbach’s α=0.82). Results. Statistically significant correlation was found between the latency and amplitude of the P300 wave (p<0.05) and the age of the subjects, whereby amplitude decreases during the course of aging and latency increases with a slope of 2.37 ms for Fz and 2.12 ms for Cz electrode site, which makes event-related potentials a method superior in monitoring cognitive aging than the Montreal Cognitive Assessment solely whose results were not correlated with the age of the subjects (r=-0.141;p=0.150). Within the examined sample, there was no statistically significant correlation between the total score of the Cognitive Reserve Index and the age of the respondents, but a positive correlation was found between the subdomain Education and age (r=0.326;p<0.001). No statistically significant correlation of ERP parameters with the total CRIq index was found, while statistically significant correlations of P300 latencies with the domain of WorkingActivities and P300 amplitude with the domain of Education were found. A highly statistically significant correlation was found between the cognitive reserve index and the results of the MoCA test, which was of a moderate degree (r=0.434;p<0.001). Based on the results of the Pittsburgh Sleep Quality Index (PSQI), it was found that the respondents had on average slightly worse sleep quality (5.70±3.44) than the recommended values (PSQI<5). Within the examined sample, there was no correlation between the age of the subjects and the total PSQI score, but it was found that poorer sleep quality was correlated to lower relative P300 amplitudes on the Cz electrode site, while the most significant correlations were between the amplitude and the Daytime dysfunction and the Use of sleeping medication. A negative correlation between the total PSQI score and the domain of Executive Functions on MoCA test was found (r=-0.196;p<0.05). The duration of napping during the day was positively correlated to the P300 latency on the vertex (r=0.210;p<0.05). The results of hierarchical regression analysis showed that the total result of the MoCA test has the most significant individual contribution to the prediction of P300 wave latencies over the both registered electrode sites (β≈-0.5;p<0.001). Individual contributions to the prediction of relative P300 amplitude on the vertex showed: age (β=-0.223;p=0.029), CRI education (β=-0.367;p=0.008) and total PSQI score (β=-0.208;p=0.027). Conclusion. The assessment of the characteristics of the P300 wave, its latency and amplitude gives a better insight into cognitive aging than the Montreal Cognitive Assessment, but for a more detailed understanding of cognitive state, complementary use of both instruments is necessary. To assess the connection between the cognitive state and the cognitive reserve, it is necessary to consider individual cognitive domains and the values of each particular domain important for the formation of cognitive reserve: Education, WorkingActivities and LeisureTime. Poorer sleep in elderly is associated with poorer executive functioning and longer periods of naps during the day are associated with longer P300 wave latencies.