Effect of masking on circadian adjustment and interindividual differences on a rapidly rotating shift schedule.

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Abstract
The aim of the present study was to define the effect of masking on the estimation of circadian adjustment of and interindividual differences between nurses on a rapidly rotating shift schedule. The phase shift of the circadian rhythm of rectal temperature was studied in 17 subjects. The following three different methods for estimating the circadian phase shift were compared: (i) cosinor analysis, (ii) a method using normative endogenous data in which the masking effects are removed ("purification"), and (iii) a method using normative endogenous data in which the masking effects are not removed. The mean phase delay during the second night shift (compared with the morning shift) was 6.3 h according to the cosinor analyses of the raw data and 6.2 h according to the cross-correlation method of the raw data. When the masking effects were removed ("purification"), the phase shift was only 1.7 h. Diurnal type did not significantly explain the differences between individuals in the amount of phase shift of the raw data but was the only significant factor, explaining 32% of the variation between individuals in the phase shift of the purified data. Masking effects on body temperature should be taken into account before any definite conclusions can be drawn about the relation between individual factors and the adjustment of the circadian rhythm of body temperature.

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