Postural Orthostatic Tachycardia Syndrome (POTS) has emerged as a frequent manifestation of autonomic dysfunction among patients with prolonged COVID-19 (Long COVID). This study aimed to validate non-invasive screening methods for POTS using calibrated smartwatches to analyze heart rate variability (HRV), emphasizing remote applications and daily activities. The research was conducted in three phases. In Phase I, 39 Long COVID patients and 22 healthy controls were evaluated using a tilt test. Results showed a significant increase in mean R-R interval (p = 0.0136), a decrease in high-frequency (HF) power (p = 0.0315), and an increase in the LF/HF ratio (p = 0.0316) in the Long COVID group, indicating autonomic imbalance with sympathetic predominance. Additionally, the autonomic effects of COVID-19 vaccines were analyzed in 29 healthy individuals. The group vaccinated with mRNA technology exhibited a greater increase in heart rate (>100 bpm) and higher HR variance (199.35 ± 107.92 ms²; p < 0.05) compared to viral vector and inactivated virus groups. A significant trend toward reduced HR variability was also observed in patients infected in more recent years (p < 0.05), suggesting a temporal impact of COVID-19 on autonomic regulation. In Phase II, 22 healthy volunteers underwent two smartwatch-based protocols simulating postural transitions. The "supine–walking–supine" protocol showed greater sensitivity in detecting HRV patterns similar to those obtained by ECG during the tilt test. In Phase III, a case study of a 25-year-old female patient showed exacerbated tachycardia (>170 bpm) during light walking (~88% of predicted maximum HR), with an LF/HF ratio of 23.6 and substantial reductions in RMSSD and SDNN, consistent with sympathetic hyperactivity and a suggestive POTS profile. The convergence between smartwatch-derived data and conventional methods reinforces the potential of wearable technologies for screening and early detection of autonomic dysfunction. This approach supports scalable monitoring in telehealth, rehabilitation, and primary care