Using the multiple psychophysiological testing for 73 Chornobyl NPP control room operators (N = 1270 subject tests) there were revealed, that the information processing effectiveness significantly decreases during night shift, whereas during morning and evening shifts it is maintaining at the constant level practically. This is caused, on the one hand, by the circadian rhythms peculiarities of the different occupationally vital performance parameters (performance speed functions deteriorate under the absence of the improvement of others at night, while at the day time there are observed the different directioned changes for the different functions), on the other hand--by the more sensitivity of the body to the fatigue and work tension at night. The significant expression of the manifestation of natural circadian rhythms in the variations of the professionally important parameters of the operators' mental activity has been connected with the increased influence of the unfavourable factors on the workers' functional body state. There are revealed the more sensitivity of the information processing speed parameters to the fatigue in comparison with an accuracy, and the more sensitivity of the mental activity accuracy to the lengthy supported level of the nervous-emotional work tension in comparison with a speed.