Variations in length of day and access to daylight may influence the chronobiological system and cause both hormonal and psychological disturbances with symptoms like insomnia, reduced ability to concentrate, extreme tiredness, and even severe depression or anxiety (Arendt, 1998; Brainard et al., 1997; Rosenthal, 1998). Although seasonal fluctuations in sadness and fatigue may afflict a large proportion of the population in countries far from the equator, people in general may not associate this with external factors. One aim of the present study was to assess the prevalence of fluctuations in sadness and fatigue in an otherwise healthy working population. Another aim was to study whether there exists a systematic relationship between these symptoms on one hand, and insufficient artificial lighting at the work place on the other. An additional assumption of the study was that the individual's personality may have a moderating influence on the fluctuations in sadness and fatigue (Jang et al., 1997). Further aims were to study the possible impact of other climatic conditions.