If you’ve been using a Fitbit tracker to monitor your sleep, then you know how important movement is to signaling when you fall asleep, wake up, or stir. But in order to determine your sleep stages—how long you spend in light, deep, and Rapid Eye Movement (REM) sleep—movement alone is not enough.

During sleep studies, “most sleep stages are scored based on brain waves,” says Michael Grandner, MD, director of the Sleep and Health Research Program at the University of Arizona in Tucson and a Fitbit sleep consultant. “Except during REM sleep when doctors are also looking for eye movements and a lack of muscle activity.”

In other words, you can’t determine light or deep sleep based on movement alone. “You need another signal,” says Grandner. “You need something else that is going on in the body that differentiates REM from other stages of sleep.”

So what’s that second signal? Here’s a hint: It’s something many Fitbit trackers can already track—heart rate!

You’ve probably heard that your heart rate slows when you fall asleep, but did you know that it doesn’t stay steady throughout the night? Research shows that heart rate variability—beat-to-beat changes in your heart rate—fluctuates as you transition between light, deep, and REM sleep.

Fitbit Alta HR, Fitbit Charge 2, and Fitbit Blaze continuously monitor your heart rate variability along with your heart rate. When you sync your tracker in the morning, your device will use the heart-rate-variability and movement data it gathered during the prior night to estimate your sleep cycles.  

The result? A way to differentiate between light, deep, and REM sleep that doesn’t require multiple sensors, cords, and an expensive stay in a sleep lab.

In addition to detecting your sleep stages, adding heart rate to the mix also allows your tracker to better pick up on restlessness and short periods of wakefulness that you may not even remember. “Every 70 to 90 minutes when you transition out of REM and back to stage 1, you hit a fragile point where you might have a brief awakening,” says Fitbit sleep consultant Allison Siebern, PhD, consulting assistant professor at The Stanford Center for Sleep Sciences and Medicine and director of Sleep Health Integrative Program at the Veterans Affairs Medical Center in Fayetteville, NC. “Some people may even have a prolonged awaking at that point.”

If you notice more awake minutes in sleep stages than in your previous sleep data, don’t worry. Sleep stages combines the time you spend awake and restless into total awake minutes. Plus restless sleep isn’t necessarily a bad thing—studies show it’s common for adults to wake up briefly between 10 and 30 times per night.

Ready to get started? Check out How do I track my sleep and then see How Your Sleep Habits Stack Up.