Financial markets undergo cycles oscillating between periods of economic growth followed by periods of recession. This is similar to the principle of sympathovagal balance in humans representing the leverages between two interactive nervous systems: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). Based on the Efficient Market Hypothesis (EMH), we conjecture that there could exist two major groups of people who invest in financial markets: 1. Those who drive financial prices to a state of abnormality, and 2. Those who try to sustain the financial prices. To support this conjecture, we introduce the financial systemic balance index (FSBI) which is used to quantify the evolution of financial stress levels in the market. Results show that periods of higher stress detected by the proposed FSBI index correspond to those with higher levels of determinism and nonlinearity, evaluated via the Delay Vector Variance (DVV) method.