Accurate assessment of mobility in bed presents challenges to clinicians and researchers alike. Mobility is traditionally assessed by either overnight polysomnograph recording or wrist actigraphy. This paper describes an alternative system for unobtrusive and continuous monitoring of sleep movements that uses load sensors installed at the corners of a bed. This work is focused on the detection and classification of clinically relevant types of movement based on the forces sensed by load cells. The accuracy of the system for detecting movement has been evaluated using data collected in a laboratory setting. We also present a comparison of the proposed system with wrist-actigraphy.