Heart rate variability (HRV) is a wide-use and noninvasive method to access the status of autonomic nervous system (ANS) and circulation system. A resting stage (as the baseline) is often introduced before the experiment stimulus. There were 35 young healthy subjects in our experiments. We found that in a normal rest, people will calm down after ten minutes. The calm status can last for at least nine minutes. This statement is based on a statistical analysis in the intervals of participants' heartbeats. We have the conclusion as follows. Before a psycho-physiological experiment designed, the suggested minimum resting time is ten-minute to make sure the participants are calmed down.