Neurocardiovascular disorders such as atrial fibrillation–related stroke, syncope, autonomic dysfunction, and post-stroke arrhythmogenesis are commonly evaluated using static risk models that estimate long-term baseline susceptibility but do not capture short-term physiological variability across interacting brain–heart systems. Emerging longitudinal data show that rhythm burden, autonomic variability, blood pressure variability, and inflammatory tone can fluctuate over time, although whether these fluctuations converge into clinically meaningful vulnerability states remains unproven. This conceptual narrative review proposes brain–heart instability windows (BHIWs) as a hypothesis-generating framework for studying temporally bounded reductions in coupled neurocardiovascular stability. The framework organizes candidate instability across autonomic, electrical, hemodynamic, thrombo-inflammatory, and neural domains. It also introduces a brain–heart measurement stack integrating continuous digital signals, structured physiologic assessments, and structural or biological anchors. Candidate BHIWs are defined as sustained or recurrent deviations from individualized baseline patterns, especially when temporally aligned abnormalities occur across at least two physiologically coherent domains. The framework is intended to complement, not replace, established risk models by supporting empirical investigation of dynamic vulnerability, cross-domain convergence, and future decision pathways such as monitoring intensity, telemetry duration, orthostatic evaluation, and therapeutic timing. Prospective validation is required before BHIWs can inform clinical management.