I recently bought a smart band and smart scales and from a simple weight or sleeping time they can show tons of different information, like BMI, Fat%, sleeping phases, and so much more.. How do they do that?
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Smart bands use a infrared sensor that looks at your heartbeat through your skin. Your heart rate changes when you’re laying down, sleeping and during your sleep cycles and the smart band company reads this heartbeat data and processes it into a lot of info.
A smart scale usually works in two ways. It measures your weight(obviously) but it also measures the electrical conductivity between your feet. Because the electrical conductivity between your feet changes depending on body fat, they can get a pretty good idea of your body fat percentage. Combine that with your height which you usually have to input yourself and a company can tell you a lot of health information from those three data sources.
Fat percent is usually done either by checking the conductivity of your body (are there metal plates that touch your skin) or by shining an infrared light into your skin. There are subtle changes that occur based on how much fat you have.
BMI is a simple calculation if it knows your height and weight. You can look that up on a chart.
Sleeping phases can monitor things like your pulse and how much movement you have while sleeping. However their accuracy can be way off. Some devices out there are little better than snake oil when it comes to sleeping phases. They are getting better with this so it is likely just a matter of time before they reach a level of accuracy that is close enough to be reliably useful. However unless you have a diagnosed sleep disorder knowing anything about your daily sleep phases is mostly meaningless fun trivia.