Define 'heart rate variability' and its significance.

Heart rate variability (HRV) refers to the variation in time between consecutive heartbeats, which is influenced by the autonomic nervous system (ANS). The ANS has two main branches: the sympathetic and the parasympathetic nervous systems. The sympathetic system generally prepares the body for 'fight or flight' responses, increasing heart rate, while the parasympathetic system promotes a 'rest and digest' state, decreasing heart rate.

The significance of HRV lies in its reflection of the body's ability to adapt to stress and its overall health. A higher HRV indicates a greater ability to adapt to stress and is generally associated with better cardiovascular health, mental well-being, and a lower risk of cardiac events. In contrast, a lower HRV can indicate less adaptability and is often linked to stress, anxiety, or various health conditions. Therefore, monitoring HRV can provide valuable insights into an individual's physical and psychological state.