Did you know you can customize Google to filter out garbage? Take these steps for better search results, including adding my work at Lifehacker as a preferred source.Besides measuring heart rate and other metrics like steps, pretty much every fitness tracker and smartwatch measures heart rate variability, abbreviated as HRV. This measurement is the basis for recovery scores and stress readings, and you can usually find it as a standalone measurement in the vitals section of your chosen app. So lets break down what this number actually measures, and how to interpret yours.HRV is heart rate variabilityHRV stands for heart rate variability, but that doesnt mean what you might think. HRV is not related to your heart rate changing throughout the day or during exercise. (Your heart beats faster when youre exercising hard, which is what heart rate zones measure. Totally different purpose.)Heart rate variability refers to how rhythmically your heart beats. If the time from one beat to the next is always the same (at a given heart rate), you have low HRV. But if some heartbeats come more closely together, and some are more spaced out, you have high HRV.This may be surprising, but you actually want your heart rate to show some variation. High HRV is considered to be better than low HRV. Your fitness app will say youre well recovered if your HRV was higher than usual last night, and it will say youre stressed or not recovering well if your HRV was low.That said, its of course possible to have a heart rate that is too variable. In a sense, thats what an arrhythmia is. When Im talking about HRV hereand when fitness apps report your HRVtheyre considering the teeny tiny differences from beat to beat in an otherwise normal(ish) healthy(ish) person.These differences are measured in milliseconds, and you wouldnt notice them if you didnt have a watch sampling data while you sleep. If you ever notice anything wildly strange about the rhythm of your heart, ask your doctor (or seek emergency medical help, if appropriate).What does a high HRV mean?High heart rate variability (that is, high for you) tends to mean that youre relatively relaxed and not too stressed. Fitness apps will bump up your readiness and recovery scores when your HRV measured during sleep is high.The variation in beats is thought to come from the push and pull of two different body systems, the sympathetic (fight or flight) and parasympathetic (rest and digest) nervous systems. Its normal for your brain and body to be balancing these needs, so this is considered a good, normal functioning.What does a low HRV mean?Low HRV tends to occur when your body is working on somethingrecovering from hard exercise, fighting off a cold, experiencing lots of stress. If your HRV measured during sleep is low, youll get lower readiness or recovery scores. I find my HRV tanks when Im sick or when Ive been through a rough time either physically or mentally. (The day after a race or a weightlifting competition is always a low HRV day for me.)What is a good HRV number?Ill give you some numbers in a minute, but the most important thing to know is that HRV is most useful when you compare readings to what is normal for you. Your HRV can be higher than mine, but still low for you, or vice versa.So please dont go around asking whether an HRV of 50 is good. Is it higher than usual for you? Then its good. Is it lower than usual for you? Then maybe its indicating that your body is under stress. If your typical HRV readings range from 35 to 50 (for example), then 50 is high for you.It doesnt matter whether the average person your age has a HRV of 20 or of 100; those numbers are just fun facts about the population in general. They dont apply to you. Studies tend to show that fitter people have higher HRV, but also that HRV declines as we age. The average mans HRV is slightly higher than the average womans, but the full range of whats normal for each gender almost fully overlaps.Theres one more important caveat: when you look up HRV data, youll find that different studies and different devices each measure HRV differently. For example, this meta-analysis found the average HRV was 42, with a range of 19 to 75, when measured as rMSSD (root mean square of successive differences).Soand remember this is just for fun, here are some graphs from two wearable companies that publish their data and they both use rMSSD:Whoops breakdown of HRV data from their users finds men have an average of 40 and women 37. They have graphs breaking down how this changes with age, and showing the variation for each group.Ouras breakdown of HRV from their users finds an average of 41 across all users, and they also graph it by age and gender.As Ive found when comparing multiple devices on HRV, devices dont always report the same HRV numbers, but usually you can follow similar trends on all of them. If youre sick, theyll all report low HRV. If you have a good day, theyll all report high HRV. So pay more attention to the trend than the specifics.