[Circadian variation in alertness, readiness for work and work efficiency].
Among various rhythmic processes with different frequencies (periods), which are present in the human organism, for normal functioning the most important are the rhythms with a cycle length of about 24 hours (termed circadian rhythms). Circadian rhythms have been confirmed at all levels of physiological functions: from subcellular and cellular mechanisms, to the organic systems and organism on the whole. In normal everyday circumstances the rhythmic processes in the human organism are synchronised with the rhythmic processes in the environment. The external rhythmic variations support internal synchronisation between various rhythms, and in that way make it possible for the organism to better adapt to the environment. The sleep--wake rhythm is inverted during night work: the worker has to work in the period of low activity level, and sleep during the day, when he is usually active and alert. In order to work in shifts the worker has to adapt to such disruptions of the sleep--wake pattern. Tolerance to shift work was found to be associated with some features of the worker. The measurement of such features could serve as determinate of interindividual differences in shift work tolerance level.