Heart rate variability refers to the variability in the timing between heartbeats, which is controlled by competing inputs from the sympathetic and parasympathetic branches of the autonomic nervous system. This variability is essential as it allows the body to respond to both activating and deactivating signals, maintaining a balanced nervous system. When heart rate variability decreases, it indicates that one input is dominating, leading to an imbalance in the nervous system. Therefore, heart rate variability is not just about the heart's fluctuations, but it signifies the responsiveness of the nervous system to a wide range of stimuli, which is crucial for overall health.