Mental fatigue is commonly studied using prolonged cognitive tasks, but physiological changes observed in single-group paradigms may also reflect time-on-task, circadian variation, nap deprivation, posture, and cognitive load. This exploratory study characterized continuous heart-rate variability (HRV) dynamics during a 60-min 2-back task performed in the habitual afternoon nap period. Twenty-six healthy graduate students completed pre- and post-task assessments using the mental-fatigue subscale of the Fatigue Self-Assessment Scale (FSAS) and a simple reaction-time test; 22 participants were included in the HRV analysis after four recordings were excluded because insufficient compliance with the required seated protocol resulted in inadequate valid data. Ballistocardiogram signals were acquired using a 1000-Hz fiber-optic sensing cushion. Nineteen HRV features were estimated in 5-min windows with a 1-min step, and temporal dynamics were described using baseline-relative change (Delta) and log change rate (LCR). The FSAS mental-fatigue score increased from 18.99±17.09 to 46.88±24.89 (mean change 27.88, 95% CI 19.34-36.43; paired t(25)=6.72, p<0.001; Cohen's dz=1.32), and reaction time increased from 371.63±37.74 to 407.62±66.70 ms (mean change 35.98 ms, 95% CI 21.17-50.80; t(25)=5.00, p<0.001; dz=0.98). A BCG-derived respiratory surrogate showed a phase effect for relative respiratory amplitude but not respiratory rate. Across the 19 HRV features included in the primary analysis, several nominal phase effects were observed, but none remained significant after Benjamini-Hochberg false-discovery-rate correction. Exploratory ranking and dynamic time-warping clustering indicated heterogeneous feature trajectories, although the fixed five-cluster solution was not the best-separated solution and participant-level reproducibility was limited. These findings support the feasibility of unobtrusive BCG for describing continuous physiological variation during prolonged cognitive engagement, but they do not establish fatigue-specific HRV phases, diagnostic markers, or individual-level monitoring performance.
