Have you ever wondered how fitness trackers track your sleep? The best fitness trackers (opens in new tab) can already deliver a wealth of health data, monitoring everything from blood oxygen levels to atrial fibrillation – but how do they track your naps? And how accurate fitness trackers are (opens in new tab) when recording your dream?
We reviewed the latest data and spoke with Justin Roethlingshofer, the founder of OWN IT—a digital platform that helps coaches interpret fitness tracking data—to find out.
How does your fitness tracker track your sleep?
Fitness trackers usually have built-in accelerometers and gyroscopes that allow them to track and monitor your movement. By analyzing your movement data throughout the day, the sensors can estimate when you are active and when you are inactive (sleeping). This process is known as actigraphy, which Roethlingshoefer describes as “a non-invasive technique used to assess cycles of activity and rest.”
However, this is not the only way trackers monitor sleep. Some devices combine acitography with photoplethysmography (PPG) to get a more accurate picture.
PPG involves the use of a light source and a photodetector, Roethslingshoefer explains. “The light source emits light to tissue [such as those found in your wrist] and the photodetector measures the reflected light from the tissue.
Because blood absorbs and reflects different wavelengths of light, the photodetector can make certain measurements from these reflections. It can use the reflected light to measure things like blood volume and oxygen saturation – but it can also assess things like heart rate and heart rate variability by analyzing how much blood is passing through the illuminated spot per second. This the data, in turn, can be used to make observations about a person’s sleep, as people’s heart rates tend to slow and oxygen levels drop at different stages of sleep.
How will you know if your tracker is using PPG? Look for several small LEDs on the back of the device.
“The most common PPG sensors use an infrared light-emitting diode (IR-LED) or green LED as the primary light source,” says Roethslinghoefer. “IR-LEDs are most often used to measure blood flow that is more deeply concentrated in certain parts of the body, such as muscles, while green light is usually used to calculate the absorption of oxygen in oxyhemoglobin – oxygenated blood – and deoxyhemoglobin – blood without oxygen.”
How accurate is sleep tracking with a fitness tracker?
Unfortunately, Roethslinghoefer says sleep tracking isn’t as accurate, especially when it comes to actigraphy. Although actigraphy can be used to guess the length of sleep, it’s not very good at detecting different stages of sleep – and it can even think you’re asleep when in fact you’re just not moving much.
Even taking into account modern developments like PPG, Roethslinghoefer estimates that the accuracy of sleep tracking is about 60%. But he also points to some recent studies that show promising improvements.
In a 2019 study published in the Journal of Medical Internet Research (opens in new tab)Fitbit, one of the world’s most popular wearable device brands, was shown to offer impressive accuracy given the entry-level price for sleep tracking, but still pales in comparison to polysomnography (PSG), which remains the gold standard for sleep tracking.
And in a 2020 study at the University of Arizona (opens in new tab) had some promising results for the Whoop 4.0 tracker. The study showed that the group predicted sleep duration to within 17.8 minutes and also had very accurate detection of REM and slow-wave (deep) sleep.
In summary, you will get more accurate sleep stage data from spending time on a sleep study that uses the PSG method, but your fitness tracker will be able to track sleep duration in a much more convenient way.
Why track your sleep with a fitness tracker?
Sleep tracking may sound strange, but it actually provides more data to use to improve your physical and mental state.
For example, a fever at night can be a sign of an impending illness. Some fitness trackers even allow you to monitor the different stages of sleep, which are:
N1 dream: The initial stage of sleep where your body begins to relax and you may feel twitches. It usually lasts up to five minutes.
N2 sleep: At this point, your heart rate will drop and you will begin to breathe more slowly. This stage can last up to 25 minutes.
Slow wave sleep (SWS): This stage is defined by delta waves in the brain and is important for growth and recovery. This stage can last up to 40 minutes in early sleep cycles and will gradually become shorter as we spend more time in REM.
Rapid eye movement (REM) sleep: During REM, your body goes into atonia, a temporary paralysis, while your eyes move rapidly even though they are closed. Your brain is most active in REM sleep, almost as much as when you are awake. REM time increases with each sleep cycle and can last about an hour.