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Frontal beta oscillations are associated with top-down control mechanisms but also change over time during a task. It is unclear whether change over time represents another control function or a neural instantiation of vigilance decrements over time, the time-on-task effect. We investigated how frontal beta oscillations are modulated by cognitive control and time. We used frontal chronic electrocorticography in monkeys performing a trial-and-error task, comprising search and repetition phases. Specific beta oscillations in the delay period of each trial were modulated by task phase and adaptation to feedback. Beta oscillations in this same period showed a significant within-session change. These separate modulations of beta oscillations did not interact. Crucially, and in contrast to previous investigations, we examined modulations of beta around spontaneous pauses in work. After pauses, the beta power modulation was reset and the cognitive control effect was maintained. Cognitive performance was also maintained whereas behavioral signs of fatigue continued to increase. We propose that these beta oscillations reflect multiple factors contributing to the regulation of cognitive control. Due to the effect of pauses, the time-sensitive factor cannot be a neural correlate of time-on-task but may reflect attentional effort.