Heart rate variability (HRV) refers to changes in the time between each. In general, variability is considered healthy and even positive as it enables people to adapt to the needs of their current setting, but can sometimes indicate other issues especially if the variations are more severe (>120ms) and are occurring without cause. HRV tends to decrease with age as older individuals bodies' tend to be less adaptive. Low HRV is associated with several negative physical and mental health outcomes.