Sleep tracking doesn’t measure anything – the graph is flat.

From various error reports we know that the accelerometer sensor implementation on some firmwares has issues. Over the time the accelerometer may get into a broken state and it stops sending data. The only way we know to resolve this issue is to do a reboot. In all cases so far, reboot did solve this issue.

To test whether the accelerometer is working, you can useĀ menu > test sensors or some 3rd party app such as Sensor List.