How do smartwatches track your sleep?

So you love your shiny new fitness tracker and want to start tracking your sleep – but how does sleep tracking really work?

Sleep tracking has become big news in recent years. With roughly one in three of us using wearables these days – whether it's a full-featured smartwatch like the Apple Watch 7 or Samsung Galaxy Watch 4, or a simpler fitness tracker like the Fitbit Inspire 2 or Huawei Band 5 – this means more of us are tracking our health in different ways.

In addition to tracking our fitness and activity levels, most of these devices can also monitor the amount of sleep we get each night, tracking our heart rate and movements to determine what stage of sleep we may be in .

The four stages of sleep

There are four main stages of sleep, which together form one cycle of about 90 minutes. During each of these stages, our body movements and heart rate differ.

"The first stage is short, lasting only between five and 10 minutes," explains Rob Hobson, nutritionist and author of The Art of Sleep. "During this stage, sleep is very light and you can be easily awakened."

"Stage two, or light sleep, is characterized by a slowing of both breathing and heart rate and makes up about half of the time we spend asleep."

Stage three is deep sleep, when your body relaxes completely and your heart rate and breathing drop even more.

"This stage is restorative and essential for recovery and growth, as well as strengthening the immune system," says Hobson.

The final stage is REM, when your body is inactive but your eyes are moving rapidly. During this stage, our heart rate increases and breathing becomes more erratic.

"Body functions, such as protein synthesis, are at a peak now," says Hobson. “It's also when you're most likely to dream. This stage is considered essential for mental functions, learning and creativity.

How does sleep tracking work?

Smart watches or fitness bracelets monitor our sleep by measuring movements and heart rate that correspond to a certain stage of sleep. While older trackers simply monitored movements, now they use much more sophisticated technology.

"Motion data was often unreliable because a person could be lying still but still awake," explains Will Turner, co-founder of fitness app GoJoe.

“New technologies have emerged that, when used in conjunction with motion data, make sleep tracking much more reliable. The main way this is achieved is through photoplethysmography (or PPG) – the blood under the skin reflects the red light emitted by your smartwatch and absorbs the green light.

With each heartbeat, the emitted green light is absorbed. In the time between each heartbeat, the amount of green light absorbed is less. This data is detected by your smartwatch and thanks to it and the movement data, the device is able to know if you are sleeping.

“Some devices also use 'pulse oximetry', which analyzes the level of oxygen saturation in your blood. This feature, generally recognized as more reliable than the "red light, green light" method, uses a combination of red and infrared sensors that react differently to blood that is more or less saturated with oxygen. Highly oxygenated blood absorbs more infrared light and allows less of the red light to be reflected back, allowing your device to analyze your sleep.

Oxygen levels in your blood drop slightly when you sleep as your breathing rate decreases. So, in theory, your tracker should be able to monitor your blood oxygen levels and use that information to confirm that you're sleeping.

Can sleep trackers improve sleep?

Right now, it's unclear—not least because everyone's sleep is different, but also because many experts believe that sleep tracking may do more harm than good for some.

"While it's great to see people paying more attention to sleep and their own sleep habits, in some cases it causes unnecessary worry," explains Lisa Artis of The Sleep Charity.

“There's even a term for it: orthosomnia. Orthosomnia describes people who obsess over the results of their fitness trackers. Unfortunately, unlike eating five servings of fruit and vegetables or exercising daily, you often can't control whether you get eight hours of sleep. And if people start putting pressure on themselves to sleep better, chances are they won't.

How accurate are sleep trackers?

The answer, unfortunately, is unclear.

"There have been several studies comparing the accuracy of sleep monitoring devices with recognized laboratory tests called polysomnography, which look at brain waves, blood oxygen levels, heart rate, breathing, and eye and leg movements," says Hobson.

"A comprehensive study published in the journal Sleep Medical Reviews found that sleep-tracking devices were only accurate 78 percent of the time when identifying sleep versus wakefulness. This accuracy dropped to just 38% when estimating how long it took a participant to fall asleep.”

One major obstacle is that people don't necessarily understand how to interpret the data from their devices—which means there's no point in getting it. "As with most things—eating, breathing, mindfulness—there's never a one-size-fits-all," adds Hobson. "These apps use algorithms that have to follow a certain way of working, so the apps will never be as effective as seeing a proper professional if you need real help."

How to improve your sleep

Whether you choose to track your sleep or not, there are some easy steps you can take to improve it.

"If you're obsessed with technology, stop using it," advises Artis. "Instead, put into practice some principles of good sleep hygiene – maintaining a regular sleep pattern, getting enough relaxation before bed, avoiding caffeine and alcohol, creating the right soothing environment in the bedroom, etc." Remember, the best way to gauge how well you sleep is to listen to your body and gauge how you feel the next day.