Lighting systems are usually one of the largest electrical end-uses in underground metro stations. Taking into account that budget restrictions in publicly owned companies hinder energy efficiency retrofit projects that require high initial investments, affordable energy saving strategies are needed. This paper presents a low-cost approach for reducing lighting electricity use in underground stations, without affecting passengers' comfort or the metro operator's service. For this purpose, an adaptive lighting strategy of dimming the illuminance levels of artificial light sources has been developed. Dimming controls are based on the occupancy of the station, and the preventive maintenance and cleaning cycles of the luminaires. The stations' monthly occupancy patterns are defined through the k-means clustering technique. To illustrate its effectiveness, the method was applied to 115 underground stations of the Barcelona metro network. The results revealed overall electricity savings of 255.47 MW h on a biannual basis, which represents 36.22% of the stations' baseline lighting consumption. Individual energy savings were found to range from 25 to 87.5 MW h/year in the stations of the Barcelona metro network, depending on the number and profile of station users. The research findings will undoubtedly be useful for the future energy efficiency project plans of worldwide metro operators and managers of other underground spaces. (C) 2016 Elsevier Ltd. All rights reserved.