This paper describes how data collected during a continuously running data acquisition program on the LESO building in Lausanne, Switzerland, was used to measure the intermediate light switch probability by users as a function of current illuminance levels, i.e. the probability for a given timestep that the user will switch on or off the electric lighting, excluding such actions that happen upon user entry to or exit from the office. We assume such a probability to be independent of the user's history and further derive some theoretical consequences of this postulate. In particular, we show how a history-less user leads naturally to patterns of behaviour already observed in real buildings. (c) 2006 Elsevier B.V. All rights reserved.