ABSTRACT
Humans seem to be the only diurnal species requested to be active at night and to sleep during the day at certain circumstances, i.e. when they work shifts including night work. Humans evolved in the rhythmic environment resulting from planetary movements (e.g. light/dark cycles). This lead to the development of internal mechanism (a body clock) producing biological rhythms of body processes and behavior (e.g. circadian rhythms) to prepare and enable humans to function in the rhythmically changing environment. The social environment exhibits rhythms structured according to daytime working hours and night time rest. How shift working people react to the displacement of natural sleep-wake pattern in the day oriented pattern of activity and rest of the society and what are the factors which determine their reaction?
