1. A body movement monitoring system comprising: a sensor, adapted to be attached to an upper part of a body of a person, the sensor comprising an accelerometer component adapted to generate signals in response to movement of the body, the signals comprising a frontal acceleration signal and a vertical acceleration signal; one or more processor circuits programmed to: filter at least a portion of the signals generated by the accelerometer component to obtain a first set of filtered signals; determine an initial estimated postural transition duration for a first postural transition based on one or more peaks in the first set of filtered signals, and for a first time window; apply one or more additional filters to a segment of the signals generated by the accelerometer component to obtain a second set of filtered signals, the segment corresponding to only a second time window that is narrower than the first time window and that is selected based at least in part on a timing corresponding to the initial estimated postural transition duration for the first postural transition; determine a revised estimated postural transition duration that is more accurate than the initial estimated postural transition duration for the first postural transition based on a location of one or more peaks in the second set of filtered signals; and derive information related to the first postural transition using the revised estimated postural transition duration and the second set of filtered signals, the information including identification of at least one of a time of occurrence, a duration, and a type of the first postural transition.