Electroencephalography (EEG) is an essential tool for assessing the neural activity behind mental fatigue, a factor that can influence both cognitive and physical performance. Mental fatigue influences alpha-band oscillations (8 to 12 Hz), reflecting cortical engagement and attention stability. In this study, EEG recordings from 109 participants in the Physionet open-source EEG Motor Movement/Imagery Dataset (EEGMMIDB) were analyzed to characterize alpha power changes during sustained attention. Spectral analysis was performed across early and late task blocks to quantify alpha desynchronization over time. Participants demonstrated a consistent decrease in alpha power ( = -0.076 +/- 0.019 log10 units; p = 0.0001), indicating desynchronization associated with sustained attentional demand. These findings show the applicability of open-source EEG datasets for research. Furthermore, these findings support the isolation of posterior activation as a marker of mental fatigue through alpha desynchronization.