We performed the first systematic search for the minimum variability time scale between 0.3 and 10 keV studying the 28 brightest early ( < 3000 s ) afterglows detected by Swift-XRT up to March 2008 . We adopt the power spectrum analysis in the time domain : unlike the Fourier spectrum , this is suitable to study the rms variations at different time-scales . We find that early XRT afterglows show variability in excess of the Poissonian noise level on time-scales as short as \sim 1 s ( rest frame value ) , with the shortest t _ { min } associated with the highest energy band . The \gamma - ray prompt emission of GRB 080319B shows a characteristic average variability time-scale < t _ { var } > \sim 1 s ; this parameter undergoes a remarkable evolution during the prompt emission ( BAT observation ) .