Steady state visual evoked potential (SSVEP) has been employed significantly in various electrophysiological studies, most notably in the brain-computer interface (BCI) designs. While the standard approach of generating SSVEP is to use a 50% duty cycle pulse width modulation, i.e. with similar on and off periods of flashing, recent advancements in LED technology have allowed higher duty cycles such as 85% to be generated. While higher duty cycles are more successful in the entrainment and hence the improved response, it is unclear whether mental fatigue is also affected by a higher duty cycle. In this paper, we evaluate mental fatigue using a beta band energy analysis of electroencephalogram in five users while they visualise conventional 50% duty cycle SSVEP and 85% duty cycle SSVEP. Our results indicate that mental fatigue reduces with increased duty cycle and also confirms the comfort ratings by subjects, who report feeling more comfortable with the higher duty cycle.