<p>Heart rate variability (HRV), regulated by the autonomic nervous system, is typically assessed using standard time-domain and frequency-domain methods to evaluate autonomic function. However, conventional linear analyses capture only a limited aspect of HRV, as the human body, including the cardiovascular system, is intrinsically nonlinear. In light of this, there has been growing interest in nonlinear analyses grounded in chaos theory and complexity science. In this study, we conducted a comprehensive comparison of time-domain, frequency-domain, and chaos/complexity indices derived from R-R interval (RRI) analysis during both physical and mental tasks. The results clearly demonstrate a significant increase in chaos/complexity indices during mental tasks, while conventional indices remain unchanged—underscoring the unique sensitivity of nonlinear measures to cognitive processes. These findings highlight the relevance of chaotic dynamics and complexity in HRV as a valuable perspective for understanding brain-heart interactions. Furthermore, based on the experimental findings, we propose a new hypothesis, consistent with previous research, regarding the emergence of chaotic features in HRV during cognitive activity.</p>