HRV stands for Heart Rate Variability, which is the difference in time intervals between separate heartbeats. HRV relates to the autonomic nervous system that regulates body functions without conscious effort. HRV can be significantly influenced by age, race, sex, physical fitness, clinical conditions, and drug treatment. But 24-hour HRV appears to be stable when measured on a day-to-day basis and over periods of days to weeks. HRV is low with (physical) stress where the heart rhythm becomes more regular. A healthy heartbeat, however, contains healthy irregularities. This means that illness, bad sleep, a bad lifestyle, dehydration, or overtraining can lower HRV. On the other hand, a clear head, a healthy lifestyle with a good night’s sleep and a balanced physical training scheme can give a clear improvement in HRV.