Attention functions form the basis of other cognitive processes and are crucial for coping with everyday situations. Individual performance within various cognitive functions is influenced by factors such as the circadian rhythm and individual chronotype. This study focuses on tonic alertness, one of four cognitive functions that were examined. The question of how circadian variations in tonic alertness behave throughout the day in extreme chronotypes is investigated. To this end, 60 test subjects, who were initially classified as morning and evening types, were tested at five different times of day. The results indicate a stable course of tonic alertness throughout the day. No significant differences in reaction times between chronotypes were found, nor were any diurnal fluctuations observed.