Circadian rhythms, sleep homeostasis, and chronotype interact to modulate cognitive processes such as attention and executive functions, leading to fluctuations in performance throughout the day and significant impairments under conditions of misalignment or sleep deprivation. The study was conducted in two phases and included the assessment of subjective measures of sleepiness, vigor, affect, sleep quality, and individual chronotype. In the first phase, 32 healthy participants (19 females; mean age = 24.9 ± 3.14) were tested at four different times of the day to examine diurnal variations in attentional performance. In the second phase, a subsample of 8 participants (4 females; mean age = 24.4 ± 2.20) underwent a total sleep-deprivation protocol and completed the AD-Task over two consecutive days to evaluate the impact of prolonged wakefulness on attentional and executive functioning. In the first phase, results revealed a progressive decline in selective attention across the day (F(3.93) = 3.188; p = 0.027; η2p = 0.093) and significantly slower reaction times in divided attention during the morning (F(3.93) = 7.3134; p < 0.001; η2p = 0.191), indicating a time-of-day modulation of attentional resources. In the second phase, total sleep deprivation led to a marked impairment in performance, reflected by slower reaction times (F(11.77) = 1.948; p = 0.046; η2p = 0.218) and reduced discriminative ability (d'; F(11.77) = 13.438; p < 0.001; η2p = 0.657), particularly in task-switching conditions. These findings suggest increased vulnerability of executive functions under sleep deprivation, likely associated with reduced efficiency of prefrontal cortical processes. The results confirm that time of the day and pressure of sleep significantly affect attention and cognitive flexibility and further affirm the validity of the AD-Task as a reliable instrument for detecting these variations under ecologically relevant conditions.
