How Do Fitness Trackers Measure Sleep?

How do fitness trackers measure sleep

I know for a fact that if I get a good night’s sleep I feel ready for the day ahead. However, I would like to know more; about my sleep and often find myself wondering how do fitness trackers measure sleep.

Indeed, as time goes, research is beginning to show that sleeping well is a massive part of maintaining a fit and healthy body. In fact, recent research has shown that lack of sleep can appear to have the same affects as being drunk or make you crave fatty calorific foods. Knowing this, the manufacturers of fitness trackers decided early on that sleep tracking was of great benefit. We are going to delve further and take a look at the science behind sleep tracking to help us understand how do fitness trackers measure sleep.

A brief science of sleep

It is a fascinating topic which in simple terms, sleep can be broken down into 2 phases. Firstly NREM (non rapid eye movement), followed by REM (rapid eye movement). Each one of these can then be further broken down into 3 phases of NREM sleep, followed by 1 NREM phase taking you all the way from when you go to sleep to just before you wake.

During each phase of sleep there are differing levels of brain activity, eye movement, breathing, blood pressure and muscle tension. Fascinating stuff and you can find a more in depth analysis here.

Those in the business of sleep research have agreed on a technique known as polysomnography to measure sleep. This is performed in lab conditions and requires the attachment of multiple wires to the subject undergoing the study to measure multiple channels of data. Clearly, this isn’t practical for a daily use fitness tracker and a different technique was required.

How do fitness trackers measure sleep

Numerous fitness on the market come with sleep tracking functionality and some even claim that they are able to track how long you spend in the different phases of sleep. The question is how?

According to Fitbit and the makes of the Mi band app the ability to measure sleep comes from the fitness tracker’s accelerometers. The software in the tracker detects movements, speed and direction of the accelerometer. If no movement is detected for a certain period of time, let’s say an hour an assumption is made that you are asleep. This is a technique know as actigraphy, each manufacturer will have  a proprietary algorithm to interpret the data. The results will be the ability to provide details of how long you sleep, when you were restless and even when you are awake

However, even with all these goodies, according to the experts this method of tracking sleep is prone to inaccuracies and people should not come to rely on any fitness tracker that tracks sleep. Aware of these criticisms Fitbit and others are putting more effort into improving the sleep tracking aspects and the Fibit Inspire 2 is adding the input from the heart rate monitor to help identify what phase of sleep you are currently in.


In our view, fitness trackers that measure sleep are useful to view long term patterns of your sleep and get a feeling for your sleeping habits but they should not be relied on. If you feel that you are suffering from any form of sleeping disorder seek the advice of a medical professional