Previous self-report based research has revealed a heightened propensity to slips-of-action in the early morning and at the end of the day. Here, we examined performance variability among healthy young adults as a function of time-of-day on a clinical task that is sensitive to absent-minded slips in brain-injured groups. We found significantly higher error rates at 1 pm and 7 pm compared with 1 am and 7 pm, and significant correlations between errors and two subjective sleepiness scales. No circadian modulation of the more routine aspects of the task was observed suggesting some specificity to the effect. Given evidence that the circadian cycle differentially affects different brain regions, and links between sleep deprivation and 'normal' dysexecutive behaviour, examining variation over the course of the day can prove a useful additional methodology in this area