The aim of the study was to investigate to what extent variations in natural daylight, as well as in indoor lighting and decoration, would affect the mood of people working indoor. It was assumed that such an impact would be moderated by the personality characteristics of the individuals. The study was carried out in northern Argentina in workplaces consisting mostly of open plan offices, but also some rooms for one or two persons, and a factory plant were included. About 20 per cent completely lacked windows. In spite of the considerable variations in day length, ranging from ten-and-a-half hours in June to fourteen hours in December, no overall seasonal variations in mood could be established. One obvious reason for this may have been the frequent use of shading devices which reduced the impact of the variations in natural daylight between summer and winter. On the other hand, the differences between the interior environments in terms of lighting and decoration did seem to have at least some impact. Both analyses of regression and variance showed interior decoration to be the most important from the emotional point of view. The participants throughout, reported a more positive mood in the colourful environments. The difference was consistent over the year but became significant only during autumn and winter. Individuals of type B (few signs of tension) reported a more positive emotional status throughout the year than those of type A, which is in line with previous results linking type A to anger and hostility.
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