We propose the spectral exponent β = D/(D+1) as a universal vital sign for complex systems operating within the Gap C∈(0,1) of the Axiomatica Universalis. Systems near their optimal operating point exhibit β ≈ D/(D+1); deviation indicates systemic stress. Three testable predictions are derived for medical diagnostics (EEG, HRV, anaesthesia depth), ecological monitoring (biosphere β-collapse at Pinatubo 1991: β 1.803→−0.256), and financial stability assessment. β is proposed as a capacity measure, not a consciousness marker.