From a medical point of view night work is of particular concern as the physiological rhythms dissociate with each shift from day to night work and back to day work, and only few workers are able to adapt fully (i.e. resynchronisation of the physiological rhythms and the altered sleeping-waking cycle). In the 1980s light was identified as the dominant factor determining the perception of time, and its chrono-biological features and the role of melatonin in this process were investigated. Experimental and field studies of shift work carried out in the 1990s led to faster adaptation to night work as a result of the direct application of light and melatonin. On the basis of these studies this article discusses whether, and under which conditions and restrictions, treatment with light or melatonin can be recommended.