Shift work creates a psychosocial working environment that may adversely influence the health of the worker. Night work, and the change of working hours from one shift to another, may subject the workers to certain stresses which affect the nervous system, thus increasing the frequency of nervous symptoms such as fatigue, nervousness, irritation and insomnia. These nervous symptoms are usually related to lack of sleep. Shift workers may have a higher incidence of chronic gastritis and peptic ulcer than day workers. The stress associated with night work was found, in a study carried out in Egypt, to contribute to a heightened incidence of high blood pressure.
Psychological and social factors, whether or not combined with physio-pathological disorders, may play a decisive role in a person's inability to adapt to shiftwork. Generally speaking, attention is drawn to the feeling of being cut off from the community and of not participating in social life and collective responsibilities. Moreover, when members of the same family have different work schedules it is particularly difficult for them to get together and organize their family life. Because of the need to work during weekends and on holidays, collective leisure activities, family and neighbourly relations and participation in group entertainment also suffer from shiftwork.
Research in the field of shiftwork has led to the conclusion that the accident rate decreases during the night while the seriousness of accidents increases. On the other hand, where environmental factors are particularly relevant, as in the case of road transport, the element of risk is much higher during the night, particularly between midnight and 4 am, than during the day. It has also been found that the mental alertness of drivers of railway locomotives declines towards 3 am in actual working conditions. Looking, for example, at the whole of the metal processing industry in France, 27.5% of the employees work three shifts and 11.7% four or more shifts. In iron and steelworks alone, 70% of the manual staff is employed on shift work. Similar figures are found in the paper industry, compared with 60% in the chemical industry, compared with 60% in the chemical industry and 45% in the glassware industry. All in all, 900 000 workers in France have to work at night.
In virtually all epidemiological studies, the most frequently observed disorders have to do with the digestive system, frequently combined with sleep anomalies. Altogether, some 25 to 30% of shiftworkers complain of lack of sleep when undergoing clinical tests, either because of difficulty in going to sleep or a tendency to wake up early. In almost 50% of the cases covered by studies of intolerance symptoms connected with shift work, lack of sleep is combined with other pathological disorders. In a survey conducted in France in 1973, of the people surveyed, 30% suffered little inconvenience from shiftwork; 25% encountered some difficulties; 35% encountered more serious problems that had repercussions either on their health or on their family life, usually because the wife also worked and 10% developed serious sleep and health disorders and found it difficult to organize their family and social life with the result that crisis situations arose that seemed to be caused by a physiological disequilibrium rendering adaptation impossible.