The intervention effect of caffeine oral tablets on mental fatigue induced by nap deprivation: based on assessments of heart rate variability, cognitive performance and subjective fatigue—a pilot study

Background This study aimed to investigate the feasibility of inducing mental fatigue (MF) through nap deprivation, based on monitoring heart rate variability (HRV), cognitive performance, and subjective fatigue levels. Additionally, the intervention effect of caffeine oral tablets (COT) on MF was evaluated. Methods This was an open-label, non-randomized, single-sequence crossover pilot study. The caffeine content in COT was quantified using high-performance liquid chromatography (HPLC). Ten healthy volunteers who habitually took naps were enrolled. Participants underwent nap deprivation to induce MF. Various parameters were collected at different time points, including the Karolinska Sleepiness Scale (KSS), Attention Network Task (ANT), and HRV metrics. Changes in each parameter before and after COT intervention were analyzed. Results Following nap deprivation, there was a significant increase in KSS scores for the control group compared to baseline measurements taken after 2 h of sleep deprivation ( P < 0.01). No significant changes were observed in ANT performance; however, HRV exhibited variations influenced by circadian rhythms—showing increased volatility in time-domain parameters and differing trends in frequency-domain parameters without statistical significance. After administration of low-dose COT to experimental participants, KSS scores did not significantly rise; conversely, correct response rates (CR) on ANT improved significantly ( P < 0.05), alongside notable changes in both time-domain and frequency-domain HRV parameters ( P < 0.05). Conclusions In this pilot study, the alignment between subjective and objective indices suggests that low-dose COT is associated with favorable changes in the MF state of volunteers following sleep deprivation. However, due to the small sample size and open-label design, these findings should be considered preliminary and warrant confirmation in larger, randomized controlled trials.