There were also significant time x exercise intensity interaction effects for heart rate variability and plasma noradrenaline ( F [ 8 , 72 ] = 2.23 , p = 0.03 ) , with a bias in noradrenaline occurring between light and vigorous conditions ( adjusted mean difference [ SE ] = 2850 ng/ml [ 592 ] , p < 0.01 ) at 5 min post-exercise .