Apparent phase-shifts of circadian rhythms (masking effects) during rapid shift rotation.

Six subjects worked an experimental 8 h rapidly rotating shift system in which 6 shifts were compressed into 5 d (work two mornings - 8 h interval - work two nights - 8 h interval - work two afternoons). Rectal temperature was continuously recorded and urinary excretion rates of adrenaline, noradrenaline, K+, Na+ and Hg+ were assessed in samples collected at 4 h intervals. Higher rectal temperatures and higher excretion rates of the two each catecholamines than those expected from the normal circadian rhythms were observed during night work, while lower than expected values were recorded forenoon sleep. In these three rhythms, immediate, significant 3-4 h delay phase-shifts occurred in the two days following night shifts. The acrophases reverted suddenly to their initial positions when the subjects returned to their normal sleep/wake schedule. The circadian rhythms of the excretion of electrolytes were not perceptibly disturbed. It is concluded that the apparent phase-shifts were due to a direct influence (masking effect) of the changes in body functions during sleep and wakefulness on the measured parameters, and that no true phase-shifts (entrainment) occurred in the endogenous circadian regulation. The evidence of such an immediate, partial adaptation to unconventional sleep/wake patterns supports the adoption of rapidly rotating shift systems which would not greatly interfere with the endogenous oscillators.