Chronotype distribution in professional rugby players: Evidence for the environment hypothesis?

ABSTRACT

Individual sport athletes have been shown to comprise unusually high proportions of morning-types (MTs) coupled with a higher prevalence of the morningness-associated PERIOD3 variable number tandem repeat (VNTR) allele, PER3⁵. The degree to which type of sport selected is influenced by either chronotype or genotype, or the extent to which sporting environment contributes to chronotype is unclear. The aim of this study was to assess chronotype and PER3 VNTR polymorphism frequencies in team sport players and non-athletic controls. South African male Super Rugby players (RUG, n = 120) and a control population of males with habitually low levels of physical activity (defined as exercise no more than twice a week; CON, n = 117) took part in this study. Participants completed the Horne–Östberg morningness–eveningness questionnaire to determine chronotype and donated buccal cell or blood samples from which PER3 VNTR genotype was established. There were more MTs in the RUG (47%) than CON group (23%, p < 0.001), more evening-types in the CON group (18%) compared to the RUG group (3%, p < 0.001), but no differences in PER3 VNTR genotype (p = 0.619) or allele (p = 0.758) frequencies. In both groups, more people carried the PER3⁴ allele (RUG: 63%, CON: 62%). Chronotype was associated with genotype in the CON (p = 0.004) but not the RUG group (p = 0.895). Unlike the individual sport endurance athletes previously studied in whom the PER3⁵ allele predominated, the PER3 VNTR genotype distribution in these team sport players was similar to that of the general population. We hypothesise that the absence of any chronotype–genotype relationship in these rugby players is because their diurnal preference is shifted towards morningness through habitual athletic behaviour.

KEYWORDS:

• Diurnal preference
• chronotype
• PERIOD3 VNTR polymorphism
• circadian
• athletes
• team sport

Acknowledgements

We thank all South African Super Rugby team players and doctors, without whose cooperation this study would not have been possible. Thanks also to Rob Henst, Rageema Joseph, Chenjerai Muchapirei, Denford Banga and Nyambura Shawa, all from the University of Cape Town who helped with recruitment of the control participants. We are grateful to the anonymous reviewers for their comments and suggestions to improve the manuscript.

Declaration of interest

The authors have no conflicts of interest to declare.

Funding

This work was funded by a grant to LCR from the University of Cape Town Research Committee. LK received funding from the University of Cape Town Equity Development Programme and the International/Refugee Scholarship for his PhD studies.

Additional information

Funding

This work was funded by a grant to LCR from the University of Cape Town Research Committee. LK received funding from the University of Cape Town Equity Development Programme and the International/Refugee Scholarship for his PhD studies.