Heart rate variability (HRV) during 30 minute\'s recovery from a 30-minute exercise was established for 6 healthy men 23-30 years old. The exercise-recovery schedule was performed at 8 circadian stages over 11 days for each subject and analyzed over consecutive 5-minute segments. The local K index for the time of the recovery was recorded as a variable for geomagnetic activity. Pooling all HRV values across the different times into recovery, the relation of HRV to local K is found to be non-linear. A quadratic model fitted to the pooled HRV values is statistically significant. The present study indicates that during recovery from exercise, the effect of geomagnetic disturbances on HRV is non-linear. A non-linear relation of HRV with respect to geomagnetic activity deserves further work to determine whether it can account for discrepant findings of an effect of magnetic storms on the incidence of myocardial infarctions, for which a decreased HRV is a predictor.