Chronotype of South African adults is affected by solar entrainment

ABSTRACT

Our daily lives are influenced by three different daily timers: the solar clock, our endogenous circadian clock and the societal clock. The way an individual’s endogenous clock synchronises to the solar clock, through either advances or delays relative to sunrise and sunset, results in a phenomenon known as diurnal preference or chronotype. South Africa uses just one time zone, but 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. It was hypothesised first that South Africans living in the east of the country may have a greater preference for mornings (more morning chronotypes) than those living in the west; and second, that this difference would not be due to genetic differences in the populations, particularly a genetic polymorphism previously shown to influence chronotype. Here, we describe and compare the distribution of chorotype and PERIOD3 variable number tandem repeat (VNTR) polymorphism frequency in eastern (n = 129) and western (n = 175) sample populations. Using the Horne–Östberg Morningness, Eveningness Questionnaire we found that there was a significantly higher proportion of morning-types in the eastern population (56.6%) than in the western population (39.4%), and there were higher proportions of neither-types and evening-types in the western population (51.4% and 9.1%, respectively) than in the eastern population (37.2% and 6.2%, respectively) (p = 0.009). There were no significant differences in distribution of the PER3 genotype (p = 0.895) and allele (p = 0.636) frequencies. Although previous studies have shown associations between chronotype and PER3 VNTR genotypes, no significant associations were observed in either the eastern (p = 0.695) or the western (p = 0.630) populations. These findings indicate that, in South African populations, longitude influences chronotype independently of PER3 genotype. The impacts of the differences in chronotype whilst maintaining the same societal temporal organisation in the eastern and western regions were not assessed.

KEYWORDS:

• Chronotype
• longitude
• PER3 VNTR
• South Africa

Acknowledgements

We are grateful to the participants, to members of the Rhythms and Blooms laboratory and Liz Kennedy who assisted in recruitment; to Dean Goldring and Theresa Coetzer of the University of KwaZulu-Natal for their hospitality and advertisement of the study; to Dale Rae for advice and to Voice of the Cape radio station, Cathy Doherty-Bigara and Cascades Mall, Jeanne Marie Greyling and Bayside Mall, Samantha Christians and Pinelands Library for their help in the recruitment process.

Declaration of interest

The authors declare that they have no conflict of interest. The authors alone are responsible for the content and writing of this article. This study was funded by a University of Cape Town Research Committee Development Grant to Laura Catherine Roden. Nyambura Shawa was a recipient of a Molecular and Cell Biology, University of Cape Town Equity Development M.Sc. scholarship.