A sleep position trainer (1) has a normal operational mode for alerting when a posture of the body detected by the position sensor (34) corresponds with an undesired body posture. The sleep position trainer further comprises a non-movement timer (t1) for timing a non-movement period of the user wearing the sleep position trainer. An output signal is send to an alert unit in case that a posture of the body detected by a position sensor (34) corresponds with a body posture when a threshold value of the non-movement timer (t1) of at least 15 minutes is exceeded.