How Do Fitness Trackers Track Your Sleep?
Monitoring when a person goes to sleep and the quality of sleep they have has needed sophisticated equipment in the past.
Now though, using the sensors that are already in many Fitness Trackers to detect steps and activity, the manufacturers can see how the signals change as we lie down (moving to horizontal) and move around (or stay still) in bed. From this data and based on loads of trials, they are pretty accurately (but not always perfectly) able to determine when we go to bed, when we are restless or get up in the night. Some are also able to estimate when we go into different sleep states and how long we spend in each state.
How exactly the data is interpreted and exactly which sensors are used are commercial secrets, so we'll never know exactly how they do it!
How is this useful? well for many its just an interesting statistic. But maybe it will show you things that you may want to try to improve, For example, how many hours do you sleep on average? could you go to bed earlier to improve this? could you drink less in the evening and need to get up less at night and get longer periods of sleep?
As always some people seem to have their tracker capturing their sleep with uncanny accuracy, others struggle to get an accurate reading. The science of "Actigraphy" is sound though and if you suffer with sleep issues and contact your Dr, chances are that you will be fitted with a dedicated wrist based sleep tracker to record whats going on. See examples of a Medical Sleep Tracker Here.