A sensor assembly for a furniture for detecting an activity of a user of the furniture comprises a light transmitter and a light receiver forming beginning and end of an optical light path connecting the light transmitter and the light receiver. The light receiver is configured to output a reception signal based on a received amount of light. The sensor assembly further comprises an evaluation circuit which is configured to generate a movement signal based on the reception signal or a signal derived from the reception signal, and an oscillating body which is movably mounted in such a way that a movement of the oscillating body results in a change in a property of the light path. The sensor assembly is arranged such that the activity of the user causes the oscillating body to oscillate.