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How to use a sleep tracker
Download a sleep tracking app for your phone.
A simple search of your phone's app store will reveal dozens of sleep tracker options, some free and some for sale. Nearly all of them, however, rely on your phone's existing accelerometer, which detects movement.
Basically, you open the app and keep your phone in your pocket when you go to bed. If you are moving little or not at all, the app records this as sleep.