A recent trial showed a negative impact of mental fatigue (MF) on subjective and behavioural outcomes. However, effects on physical performance outcomes were smaller than previously reported, possibly due to interindividual differences. To explore these differences, we examined whether sleep quality and chronotype modulate subjective and behavioural responses to MF. Mental fatigue was induced using a 45 minute Stroop task, while the control condition consisted of watching a documentary of equal duration. Experiments were conducted at approximately the same time of day across participants. Dependent variables included the level of subjective MF (visual analogue scale), cognitive performance (GoNoGo task), physical performance (time trial) and the Rating of Perceived Exertion (during the time trial). Sleep quality (Pittsburgh Sleep Quality Index) and chronotype (Single Item Chronotype scale) were added as between subject variables in exploratory moderation analyses. Weekly caffeine consumption was included as a covariate in the analyses. Most analyses revealed no meaningful differences in MF response between chronotype or sleep-quality groups. Any differences that were found were focused within the subjective variables, with a significant influence of sleep quality (p = 0.028) and chronotype (p = 0.002) on the general subjective feeling of MF and a significant difference between chronotype groups for the feeling of sleepiness (p < 0.001). Future research should incorporate multidimensional subjective and objective measures of sleep and include athlete specific populations to further clarify the distinction between MF and impaired sleep quality. TRIAL REGISTRATION: The study was registered on ClinicalTrials.gov (NCT05576935).
