[Effect of double-shift work on the circadian dynamics of rate of heart contraction].

Circadian variations of the heart rate of 58 male workers, aged 18-21, were investigated. The builders were from different geographical areas and their work record at the construction site was 2 months, 1 or 2 years. Some of the test subjects worked only in the daytime (8 a.m. to 5 p.m.) and others worked in two shifts (day and night-5 p.m. to 1 a.m.), with the two shifts alternating weekly. Physical examinations were performed next day after the working week (those who worked in two shifts were as a rule examined after night shift). Heart rate was determined from ECGs (recorded in the II standard lead at 2-hour intervals during 24 hours; at night electrode fixation naturally disturbed the test subjects). The builders whose work record was 2 months showed an inverted heart rate: normal increase in the daytime and decrease at night was reversed and night values were greater than daytime values. This can be considered as a manifestation of anxiety due to an early change in the social, geographical and everyday environment. The builders whose work record was 1 or 2 years did not show such changes. The workers who worked in two shifts showed larger amplitudes in the circadian rhythm of heart rate, irrespective of their work record. This can be regarded as a manifestation of stress due to the night shift or continuous changes from one shift to the other.