IN THIS AGE of fitness trackers, we now have easy access to our heart rate at any given moment. Considering your heart beats almost once a second, that's a lot of data to collect. But, what do those numbers mean for your health?

Surprisingly, your resting heart rate (or, the number of times your heart beats per minute while your body is at "rest"—that's when you're sitting or laying down, having not done too much physical activity for a few minutes) can tell you a lot about the state of your health. And, it's an extremely easy metric to measure, with or without a smart watch.

It makes sense that it's telling, considering all your other organs rely on the oxygen in the blood that is pumped by your heart, says Suneet Singh, M.D., an emergency medicine physician and medical director at CareHive Health in Austin, Texas.

So, what’s a normal resting heart rate, and what can it tell us about our health? Here’s what you need to know.

What is a normal resting heart rate?

A NORMAL RESTING heart rate is between 60 and 100 beats per minute.

Having a heart rate in that sweet spot is important because it decreases the demand on your heart muscle. That means it doesn’t have to work as hard as it would if it were out of that zone, says Kate Traynor, M.S., R.N., director of Cardiovascular Disease Prevention Center at Massachusetts General Hospital.

“Think of your heart as a car and your blood's oxygen as the gas. The faster you drive, the more gas you use [the more blood that needs to be pumped]. More gas means more work for the heart, which can put it in constant overdrive,” says Traynor.

What is considered a slow or fast heart rate?

Fast Resting Heart Rates

A heart rate that averages above 100 beats per minute is called tachycardia. You can develop a high heart rate because of things like fever, anemia, dehydration, or physical or emotional stress, which triggers the release of the fight-or-flight hormone adrenaline.

"Adrenaline is like gasoline on a fire for heart rate," says Traynor. It can also lead to bigger problems—everything from fainting spells to more serious issues like blood clots that lead to stroke, or eventual heart failure (Here's how to know if you have a blood clot).

Some research found that people with a resting heart rate at or above 84 beats per minute over the span of five years were 55 percent more likely to die of heart disease than were those with lower resting heart rates.

Slow Resting Heart Rates

On the other hand, a resting heart rate below 60 beats per minute is called bradycardia, and can cause insufficient blood flow to the brain.

"An abnormally low heart rate can lead to symptoms such as feeling tired, lightheaded, dizzy, and may even cause loss of consciousness," says Suneet Mittal, M.D., F.H.R.S., of the Heart Rhythm Society.

There are some conditions, such as thyroid disease, that can affect how fast your heart beats, Dr. Singh says. “For people with an overactive thyroid, called hyperthyroidism, the excess amount of thyroid hormone can elevate the heart rate,” he explains. “Conversely, people with an underactive thyroid, called hypothyroidism, can have slower heart rates.”

Some medications are also known to affect the heart rate, Dr. Singh adds. Stimulants such as pseudoephedrine, a common ingredient found in decongestants, can elevate it. Beta-blockers, which are medications used to treat high blood pressure and hyperthyroidism, can act on heart rate as well and cause it to read as lower. Electrical abnormalities in the heart's pathways can also lower your resting heart rate.

But a low rate resting heart rate isn’t always a bad thing. Endurance athletes—say, cyclists or runners—can have rates below 40 beats per minute. This is because they are able to keep up with their basic metabolic requirements without the need for the heart to pump as much as the average person, Dr. Singh explains. "The heart learns to pump blood more efficiently when we exercise, which is why we recommend cardiovascular exercise," he says.

Without overdoing it, one of the best things you can do to maintain a healthy resting heart rate is exercise. You should be incorporating both cardio and weights into your routine, for a total of 150 minutes per week, says Traynor.

How can you find out your resting heart rate?

FITNESS TRACKERS WITH heart rate monitors can be surprisingly accurate for determining resting heart rate. But you don't have to rely on technology to get your numbers.

"The best way to determine your resting heart rate is to learn to take your pulse,” says Dr. Mittal. “This can be taken by palpating the pulse at your wrist or neck.”

Here's how to do it: Sit or lie down for about 5 minutes to allow your body to get into a relaxed state. Place your index and third fingers on your neck to the side of your windpipe. If you want to check it at your wrist, place two fingers between the bone and the tendon, looking for your radial artery—which is located on the thumb side of your wrist.

Once you find your pulse, count the number of beats in 15 seconds, then multiply that number by 4 to calculate your beats a minute, according to the Mayo Clinic.

While your heart rate may vary, it's important to keep a healthy base rate. Once you know what that is for your body, keep tabs. If you start to notice changes with your heart rate, you should check in with your doctor, especially if you notice it consistently dipping way below your normal resting heart rate, or have frequent episodes of unexplained fast beating.

"If you're a regular exerciser, but start to notice your routine takes more effort, or if you're breathless or more tired than normal during your workout, it's time to see a doctor," says Traynor.

Anyone who has concerns regarding possible problems with their heart rate should seek medical attention, Dr. Singh adds. A fast heart rate will generally present as a pounding or racing heart sensation, leading you to feel generally unwell. Slow heart rates do not create any symptoms within the chest, but instead, cause people to feel weak and dizzy. In an extreme state, a low heart rate will lead to fainting or near-fainting spells. The bottom line: If you notice any of these changes, go get checked out.

What factors affect resting heart rate?

Fitness Level

The more fit you get, the more fit your heart gets. Those who are in good cardiovascular shape train their heart to work more efficiently when they’re exercising. In turn, the heart learns to pumps more blood at a stronger force, which means it can slow down the pace at which it pumps.

Body Size

When you have obesity, your heart has to work harder to push blood flow to all areas. So, obesity can be a cause of an increased resting heart rate. Your height may be a factor too—those who are taller have a lower resting heart rate than those who are short.

Physical/Mental State

Depending on where you are, and how you're positioned, your resting heart rate may change. If you’re sitting up, your heart will pump a little faster than it would if you were laying down. When you’re vertical, your body has to fight the force of gravity to get blood to your brain. It will do this by increasing your heart rate, even if you’re not moving. External temperature affects your heart rate too—the warmer it is outside, the warmer your body temperature will be. When our body temperature warms up, our heart begins to speed up.

Your heart rate will also be dependent on your mental state. Emotions are a big predictor of your resting heart rate. If you’re particularly anxious or upset, your heart rate may reflect those feelings.

Since resting heart rate is individual and a normal rate is a range, not a number, it's worth getting to know yours so you can get things checked if it starts changing in unusual ways.