The risk of suffering anxiety disorders is associated with sustained subthreshold symptoms of anxiety. This study evaluated the stability of anxiety scores (high, moderate or low) across a six-month period in early adolescents (N = 95). The associations between sustained anxiety, vagally-mediated heart rate variability (vmHRV), sympathetic activity, and heart rate fractal dynamics in everyday life conditions were analyzed. The anxiety scores from 71.50% of participants remained at the same level. The linear correlations between anxiety and cardiac measures were weak but a group-based approach revealed that the fractal dimension (FD) from stable-low anxiety participants was higher than the FD from participants with stable-moderate anxiety scores but not higher than the FD from the stable-high anxiety group. The short-term correlations' exponent a1 from the stable-high anxiety group was higher than the a1 from the stable-moderate anxiety group but not higher than the exponent from the stable-low anxiety group. No differences were found in the vmHRV nor sympathetic activity. The lack of a direct association between the complexity of the heart rate and the level of sustained anxiety suggests a nonlinear pattern of associations that would be in accordance with the optimum variability principle.