Statistical analyses of the relation between the amount of working hours and impairments to health, based on data from a European survey on working conditions in 2000, clearly reveal that there is a substantial correlation between the number of working hours per week and the frequencies of health complaints. This applies to both musculo-skeletal disorders as well as to psycho-vegetative complaints. The relationship of the duration of the exposure to working conditions to health impairments is moderated by a great number of individual (e.g., age) and situational (e.g., shift-work) variables, showing additive or interactive effects for which selected examples have been presented. In general, however, there is a consistent functional relationship between the number or working hours and their effects on the workers that holds over a great variety of conditions. It is argued that requests for extending working hours should thus be handled with care.

ObjectiveTo examine whether any impairments in health and social lives can be found under different kinds of flexible working hours, and whether such effects are related to specific characteristics of these working hours. Methods Two studies -a company based survey (N=660) and an internet survey (N=528) -have been conducted. The first one was a questionnaire study (paper and pencil) on employees working under some 'typical' kinds of different flexible working time arrangements in different companies and different occupational fields (health care, manufacturing, retail, administration, call centres). The second study was an internetbased survey, using an adaptation of the questionnaire from the first study. ResultsThe results of both studies consistently show that high variability of working hours is associated with increased impairments in health and well-being and this is especially true if this variability is company controlled. These effects are less pronounced if variability is self-controlled; however, autonomy does not compensate the effects of variability. ConclusionsRecommendations for an appropriate design of flexible working hours should be developed in order to minimize any impairing effects on health and psychosocial wellbeing; these recommendations should include -besides allowing for discretion in controlling one's (flexible) working hours -that variability in flexible working hours should be kept low (or at least moderate), even if this variability is self-controlled. Resumo Objetivo Investigar se ocorre prejuízo à saúde e à vida social com diferentes tipos de horas de trabalho flexíveis e se há relação entre estes efeitos e características específicas das horas de trabalho. Métodos Foram realizados dois estudos, uma pesquisa em uma empresa (N=660) e outra pela Internet (N=528). O primeiro estudo consistiu de um questionário (papel e lápis) aplicado a funcionários sujeitos a diferentes ajustes "típicos" de horas de

Recent studies on flexible working hours show at least some of these working time arrangements seem to be associated with impairing effects of health and well-being. According to available evidence, variability of working hours seems to play an important role. The question, however, is how this variability can be assessed and used to explain or predict impairments. Based on earlier methods used to assess shift-work effects, a time series analysis approach was applied to the matter of flexible working hours. Data on the working hours of 4 week's length of 137 respondents derived from a survey on flexible work hours involving 15 companies of different production and service sectors in Germany were converted to time series and analyzed by spectral analysis. A cluster analysis of the resulting power spectra yielded 5 clusters of flexible work hours. Analyzing these clusters for differences in reported impairments showed that workers who showed suppression of circadian and weekly rhythms experienced severest impairments, especially in circadian controlled functions like sleep and digestion. The results thus indicate that analyzing the periodicity of flexible working hours seems to be a promising approach for predicting impairments which should be investigated further in the future.

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