Polar s810 can measure your heartbeat intervals. The differences in these periods reflect your autonomic nervous system when it regulates your heartbeat. This measurement is most useful at rest or at low intensities, because we are talking about very small differences (msecs.) Your relaxation rate indicates the state of your physical recovery. If you are overtrained, it might very well be reflected on the Polar s810 monitor.

Read the article about Polar s810 and heart rate variability