A modular rocking device (10) which can be used in conjunction with an existing bed or cot (9), without structural alteration of the same, to enable the later to be rocked. The modular rocking device (10) comprises two motion imparting legs (1, 3), two passive legs (2), two pairs of connecting rods (4, 5) and a motion and communication control unit (6). The oscillatory motion is generated within the supporting legs (1, 3) in a horizontal plane using an actuator mechanism which comprise a worm gear reducer (1.2, 1.4), or a timing pulley gear reducer (3.7, 3.12, 3.13). Motion sensors (7, 16), temperature, heart rate, blood pressure sensors (12), brain activity sensors (15), and weight sensors (2.40) detect the user presence and sleep status, and activate the rocking motion using an automated method. The motion parameters are user configurable through a control panel (6), or remotely from a terminal (13) connected to a data communication network (11).