Chronotype in the South African population: the influence of longitudinal location

Master Thesis


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University of Cape Town

Most human beings experience the pull of three different daily timers, the solar clock, their endogenous circadian clock and the societal clock. Solar time is generated by the Earth’s revolution on its axis, resulting in its surface being alternately exposed to and shielded from the sun every 24 hours. The endogenous clock, or circadian oscillator, is driven by a network of transcriptional translational feedback loops, and has a period of close to 24 hours. The circadian oscillator is synchronised to the 24 hour light-dark cycle of the solar clock. The third timer is the standardised societal clock that organises and schedules work, school, transport, appointments and free time in a 24 hour period. The way an individual’s endogenous clock synchronises to the solar clock, through advances or delays relative to sunrise and sunset, results in a phenomenon known as diurnal preference or chronotype. A person may have a morning-chronotype, where they enjoy rising and being active early in the day, an evening-chronotype where they prefer to be active later in the day into the late night, retiring in the early morning hours, or have no strong preference for early or late rising. This renders it easy for some to cope with the demands of the societal clock and others to struggle. Chronotype has both genetic and environmental influences. As society’s schedule is governed by the standardised clock, it was hypothesised that chronotype may be influenced by one’s longitudinal location within a time zone. South Africa presents an interesting case because although it uses just one time zone, in the most Easterly regions of the country, the sun rises and sets up to an hour earlier than in the most Westerly regions throughout the year. Sunrise times have an impact on the way the endogenous clock synchronises to the solar clock. It was hypothesised firstly, that South Africans living in the East of the country may have a greater preference for mornings (more morningchronotypes) than those living in the West; and secondly, that this difference would not be due to genetic differences in the populations, particularly two gene polymorphisms previously shown to influence chronotype. Therefore the aims of this study were to describe and compare the distribution of chronotype in Eastern (n=222) and Western (n=205) sample populations with the use of a validated tool, the Horne–Östberg Morningness, Eveningness Questionnaire. Secondly to describe the genotype and allelic frequency distributions of the PER2 single nucleotide polymorphism (SNP) G3853A (rs934945) in the Eastern (n= 184) and Western (n=186) populations, and the PER3 variable number tandem repeat (VNTR) polymorphism in the Eastern (n=143) and Western (n=176) populations from buccal cell samples. There was a significantly higher proportion of morning-types in the Eastern population (60.6%) than in the Western population (40.5%) (p<0.001). Whereas there were higher proportions of neither-types and evening-types in the Western population (50.8% and 8.7% respectively) than in the Eastern population (35.1% and 4.3% respectively) (p<0.001). There were no significant differences in distribution of the PER2 genotype (p=0.121) and allele frequencies (p=0.051) between the Eastern and Western populations nor in the PER3 genotype (p=0.879) and allele (p=0.075) frequencies. Although previous studies have shown associations between chronotype and PER2 G3853A and PER3 VNTR genotypes, no significant associations were observed in either the Eastern (PER2 p=0.769; PER3 p=0.221) or the Western (PER2 p=0.584; PER3 p=0.733) populations. These findings indicate that, in South African populations, longitude influences chronotype independently of genotype. Factors that may contribute to this may be the difference in the rising times of the sun, which is exacerbated to some extent by the study areas being at dissimilar latitudes and thus experiencing slight differences in climate. The impact of the differences in chronotype but the maintenance of the same societal temporal organisation in the Eastern and Western regions were not assessed. However, they may be revealed by investigating certain general health indicators in such as quality of sleep and prevalence of depressive symptoms which are affected when there is incongruence between societal time and endogenous time.

Includes bibliographical references.