In this episode of the Critical Oxygen podcast, Dr. Marco Altini and I discuss heart rate variability (HRV) and its applications in optimizing physiology and athletic performance. HRV is a measure of the variation in heartbeats over time and is influenced by the autonomic nervous system's response to stress. Dr. Altini emphasizes the importance of measuring HRV consistently and understanding individual baseline values and trends. He recommends measuring HRV every day if possible, but at least three to five times per week. When choosing an HRV device, it is important to consider the accuracy and reliability of the measurements. Dr. Altini also highlights the significance of stability and resilience in HRV, rather than focusing solely on higher values. Finally, we discuss strategies for improving HRV, including prioritizing sleep, managing stress, and making lifestyle modifications.