HRV

HRV measures how much the time between consecutive heartbeats varies. It is influenced by the autonomic nervous system, which adjusts your heart rate in response to stress, rest, breathing and more.

Many wearables report HRV, usually overnight. It tends to be affected by sleep, alcohol, illness, stress, hard training and age, and it differs a lot between people. For that reason, compare yourself with your own usual range over weeks, not with other people's numbers.

HRV is not a diagnostic test, and devices vary in how they measure it. A single low reading means little. Treat it as one piece of context alongside how you feel, and speak to a doctor if you have symptoms such as palpitations, chest pain or fainting.