THE GENETIC BACKGROUND OF INDIVIDUAL VARIATIONS OF CIRCADIAN-RHYTHM PERIODS IN HEALTHY-HUMAN ADULTS

As a group phenomenon, human variables exhibit a rhythm with a period (tau) equal to 24 h. However, healthy human adults may differ from one another with regard to the persistence of the 24-h periods of a set o f variables' rhythms within a given individual. Such an internal desyn chronization (or individual circadian dyschronism) was documented duri ng isolation experiments without time cues, both in the present study involving 78 male shift workers and in 20 males and 19 females living in a natural setting. Circadian rhythms of sleep-wake cycles, oral tem perature, grip strength of both hands, and heart rate were recorded, a nd power-spectra analyses of individual time series of about 15 days w ere used to quantify the rhythm period of each variable. The period of the sleep-wake cycle seldom differed from 24 h, while rhythm periods of the other variables exhibited a trimodal distribution (tau = 24 h, tau > 24 h, tau < 24 h). Among the temperature rhythm periods which we re either <24 h or >24 h, none was detected between 23.2 and 24 h or b etween 24 and 24.8 h. Furthermore, the deviations from the 24-h period were predominantly grouped in multiples of +/-0.8 h. Similar results were obtained when the rhythm periods of hand grip strength were analy zed (for each hand separately). In addition, the distribution of grip strength rhythm periods of the left hand exhibited a gender-related di fference. These results suggested the presence of genetically controll ed variability. Consequently, the distribution pattern of the periods was analyzed to elucidate its compatibility with a genetic control con sisting of either a two-allele system, a multiple-allele system, or a polygenic system. The analysis resulted in structuring a model which i ntegrates the function of a constitutive (essential) gene which produc es the exact 24-h period (the Dian domain) with a set of (inducible) p olygenes, the alleles of which, contribute identical time entities to the period. The time entities which affected the rhythm periods of the variables examined were in the magnitude of +/-0.8 h. Such an assembl y of genes may create periods ranging from 20 to 28 h (the Circadian d omain). The model was termed by us ''The Dian-Circadian Model.'' This model can also be used to explain the beat phenomena in biological rhy thms, the presence of 7-d and 30-d periods, and interindividual differ ences in sensitivity of rhythm characteristics (phase shifts, synchron ization, etc.) to external (and environmental) factors.