Switching behaviour is important in evaluating energy requirements of manual lighting control strategies and in understanding occupant reactions to daylight. Switching behaviour was observed and monitored in south- and north-facing private offices. The occupants appeared to fall into two different behaviour groups: (a) people who typically switched lights on for the duration of the working day, including unoccupied periods, and (b) those who used electric lighting only when daylight levels were low. Switching probability functions for two north-facing private offices were consistent with Hunt's function for multi-occupant spaces.