Night and Shift Work and Transmeridian and Space Flights

Measurements of physiological, biochemical and psychological variables at two or more different times of day reveal substantial inter-individual differences. This paper reviews studies which have dealt with these differences in terms of the morningness-eveningness, personality (introversion-extraversion), age or sex of their subjects. Studies of individual differences in the response of the circadian system to disturbance (e.g. shift work) are also discussed. The most reliable differences were observed in association with the morningness-eveningness factor. From the studies reviewed here it appeared that several rhythm parameters covaried consistently as a function of morningness-eveningness, suggesting underlying differences in the intrinsic period of the circadian system. It is argued that the differences in rhythm parameters associated with the personality dimension of introversion-extraversion are the result of exogenous influences. The results with regard to age-related and sex-related differences were not sufficient to allow conclusions to be made.

Human circadian rhythms were once thought to be insensitive to light, with synchronization to the 24-hour day accomplished either through social contacts or the sleep-wake schedule. Yet the demonstration of an intensity-dependent neuroendocrine response to bright light has led to renewed consideration of light as a possible synchronizer of the human circadian pacemaker. In a laboratory study, the output of the circadian pacemaker of an elderly woman was monitored before and after exposure to 4 hours of bright light for seven consecutive evenings, and before and after a control study in ordinary room light while her sleep-wake schedule and social contacts remained unchanged. The exposure to bright light in the evening induced a 6-hour delay shift of her circadian pacemaker, as indicated by recordings of body temperature and cortisol secretion. The unexpected magnitude, rapidity, and stability of the shift challenge existing concepts regarding circadian phase-resetting capacity in man and suggest that exposure to bright light can indeed reset the human circadian pacemaker, which controls daily variations in physiologic, behavioral, and cognitive function.