## 2 Materials and Methods
### Experimental Protocols and Data Acquisition  
The participants completed two experimental sessions held at the same time, one week apart. To minimize external factors influencing the results, the participants refrained from smoking and eating for 6 h before each session, avoided caffeine consumption for 12 h, and abstained from alcohol consumption for 24 h. The 12 h caffeine restriction was set according to its half-life: approximately 3-5 h.  
During the measurements, all participants were seated on a fixed chair in the same position, with the back of their heads touching the wall. Velcro tape and a polyester band were placed around their foreheads to secure their heads in place and prevent any movement away from the wall. A diagnostic ultrasound system (E-CUBE i7, ALPINION MEDICAL SYSTEMS Co., Ltd., Seoul, Republic of Korea) and a linear transducer array (L3-12T, ALPINION MEDICAL SYSTEMS Co., Ltd., Seoul, Republic of Korea) were used to locate and mark the right CCA for consistent measurements. The ultrasound measurements were performed by a radiologic technologist with experience in multiple ultrasound-related studies. BP was monitored using a smartwatch (SM-R850; Samsung Electronics, Suwon, Republic of Korea) worn on the left wrist and calibrated individually according to the manufacturer's instructions. The HR and peripheral oxygen saturation (SpO\({}_{2}\)) were measured using a patient monitor (Bionics, Chuncheon, Republic of Korea), with sensors attached to the left index finger. To assess cognitive performance, the participants practiced the N-back test once before the experiment to familiarize themselves with the task. The two-back (2B) test was then conducted using the DMDX software (DMDX version 6, University of Arizona Psychology Department, Tucson, AZ, USA). In this test, numbers ranging from 0 to 9 appeared randomly on the screen 40 times at 3 s intervals. Participants were required to indicate whether the current number matched the one presented two trials earlier by selecting \"positive\" or \"negative.\" \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Age (Years)** & **Height (cm)** & **Weight (kg)** & **BMI (kg/m\({}^{2}\))** \\ \hline Males (\(n\) = 15) & 22.93 \(\pm\) 1.33 & 174.93 \(\pm\) 6.25 & 74.73 \(\pm\) 11.27 & 24.35 \(\pm\) 2.94 \\ \hline Females (\(n\) = 15) & 22.53 \(\pm\) 1.36 & 159.72 \(\pm\) 3.22 & 53.38 \(\pm\) 5.62 & 20.89 \(\pm\) 1.75 \\ \hline Total (\(n\) = 30) & 22.73 \(\pm\) 1.33 & 167.33 \(\pm\) 9.15 & 64.06 \(\pm\) 13.94 & 22.63 \(\pm\) 2.96 \\ \hline \hline \end{tabular} Abbreviations: BMI, body mass index; SD, standard deviation.  
\end{table} 
Table 1: Participant information (mean \(\pm\) SD). Subjective measures were evaluated using the visual analog scale (VAS) for comfort, Karolinska Sleepiness Scale (KSS) for drowsiness, and the Chalder Fatigue Scale (CFS) for fatigue. The VAS ranges from the most comfortable state (0) to the most uncomfortable state (10), whereas the KSS ranges from highest alertness (1) to greatest sleepiness (9) [26; 27]. The CFS uses a 14-item Likert scale (eight items for physical fatigue and six for mental fatigue), with responses scored as better than usual (0), same as usual (1), worse than usual (2), and much worse than usual (3), yielding a total score of 0-42, with higher scores indicating greater fatigue [28]. Physiological and biosignal measurements were taken at three time points: before caffeine ingestion (0 min), 30 min after ingestion (30 min), and 60 min after ingestion (60 min), based on caffeine's approximate absorption time of 45 min in the bloodstream [29]. Brightness mode (B-mode) and Doppler-mode ultrasound images of the right CCA were recorded for 5 s, capturing eight cardiac cycles (Figure 1). Blood pressure (BP) was measured concurrently using a smartwatch. Participants consumed caffeine immediately following the 0 min measurement period.