One of the often-highlighted features of the new iteration of the Apple Watch is its ability to track your resting heart rate (I mentioned it in my own review of the device). Fitbit, Garmin, Samsung, and other tech brands also offer fitness trackers that measure your heart rate during exercise and keep tabs on your resting heart rate throughout the day.

We know that heart rate during exercise can show you how intensely you’re working—a higher heart rate means you’re putting greater demands on your cardiovascular system, and therefore, your heart is working really hard to pump blood quickly enough to meet those demands. The importance of resting heart rate is a little more vague, but this number can actually give you some insight into your fitness level and how it’s changing over time.

In addition, it could give your doctor information that, combined with other factors, may signal a heart problem.

Now, it’s important to first note that you should never rely on a fitness tracker to diagnose a potential heart condition. Only a doctor can make a diagnosis (don’t diagnose yourself based on what your tracker says). Trackers are not medical devices and the accuracy from tracker to tracker varies greatly. Wrist heart rate monitors can be used to glean insights into your stats, but are never a substitute for seeing a doctor and getting advanced screenings done if you are concerned that something might be wrong. That being said, these devices may be helpful in spotting overall trends and big picture changes (like if your HR is usually really low and now it’s been really high, plus you’ve felt a little dizzy or short of breath lately) that are worth talking to your doctor about more.

Your resting heart rate is a measure of how many times your heart beats per minute while at rest.

A resting heart rate between 60 and 100 beats per minute (BPM) is considered normal, but 60 to 80 is optimal, Nieca Goldberg, M.D., cardiologist and director of the Joan H. Tisch Center for Women’s Health at NYU Langone Health, tells SELF. Generally, a lower resting heart rate indicates more efficient heart function and greater cardiovascular health—and research has connected a higher resting HR with a higher risk of cardiac events like stroke and heart attack.

The best time of day to check your resting heart rate is right in the morning after you’ve woken up, George Welch, M.D., cardiologist at Manhattan Cardiology, tells SELF. “During the day, changes in your activity level, body position, emotional state, caffeine intake, and hydration levels all will affect your HR.”

Many factors influence what’s normal for any one person. Genetics, age, and gender all have an impact on your baseline HR and play a part in determining your normal range. Those aren’t really things that you can change, but there’s one factor you can: your fitness level.

Generally, a lower resting heart rate indicates a higher level of cardiovascular fitness.

“The more you exercise and the harder you train, the lower your resting heart rate,” Welch says. That’s why resting heart rate is often cited as a good measure of how fit a person is.

Greg Wells, Ph.D., assistant professor of kinesiology at the University of Toronto and author of The Ripple Effect: Eat, Sleep, Move and Think Better, tells SELF that a normal resting heart rate for an elite marathoner is more around 40 BPM. Or, for us non-Olympic athletes, Goldberg says that “instead of having a resting HR in the 70 to 80 range, with regular exercise you can get it into the 60s or below.” That’s thanks to the training effect that cardio exercise has on the heart.