Study objectives Chronotype, or diurnal preference, refers to behavioral manifestations of the endogenous circadian system that governs preferred timing of sleep and wake. As variations in circadian timing and system perturbations are linked to disease development, the fundamental biology of chronotype has received attention for its role in the regulation and dysregulation of sleep and related illnesses. Family studies indicate that chronotype is a heritable trait, thus directing attention toward its genetic basis. Although discoveries from molecular studies of candidate genes have shed light onto its genetic architecture, the contribution of genetic variation to chronotype has remained unclear with few related variants identified. In the advent of large-scale genome-wide association studies (GWAS), scientists now have the ability to discover novel common genetic variants associated with complex phenotypes. Three recent large-scale GWASs of chronotype were conducted on subjects of European ancestry from the 23andMe cohort and the UK Biobank. This review discusses the findings of these landmark GWASs in the context of prior research. Methods We systematically reviewed and compared methodological and analytical approaches and results across the three GWASs of chronotype. Results A good deal of consistency was observed across studies with 9 genes identified in 2 of the 3 GWASs. Several genes previously unknown to influence chronotype were identified. Conclusions GWAS is an important tool in identifying common variants associated with the complex chronotype phenotype, the findings of which can supplement and guide molecular science. Future directions in model systems and discovery of rare variants are discussed.

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Genetic Basis of Chronotype in Humans: Insights From Three Landmark GWAS

https://www.cell.com/current-biology/pdf/S0960-9822(14)01639-X.pdf

The Impact of Circadian Phenotype and Time since Awakening on Diurnal Performance in Athletes

Many variables related to sport have been shown to have circadian rhythms. Chronotype is the expression of circadian rhythmicity in an individual, and three categories of chronotype are defined: morning types (M-types), evening types (E-types), and neither types (N-types). M-types show earlier peaks of several psychophysiological variables during the day than E-types. The effect of chronotype on athletic performance has not been extensively investigated. The objective of the present review was to study the effect of chronotype on athletic performance and the psychophysiological responses to physical activity. The present review adheres to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) reporting guidelines. We searched PubMed, Scopus, and Web of Science for scientific papers using the keywords “chronotype”, “circadian typology”, “morningness”, and “eveningness” in combination with each of the words “sport”, “performance”, and “athletic.” Relevant reference lists were inspected. We limited the search results to peer-reviewed papers published in English from 1985 to 2015. Ten papers met our inclusion criteria. Rating of perceived exertion and fatigue scores in relation to athletic performances are influenced by chronotype: M-types perceived less effort when performing a submaximal physical task in the morning than did N- and E-types. In addition, M-types generally showed better athletic performances, as measured by race times, in the morning than did N- and E-types. Other results concerning chronotype effect on physiological responses to physical activity were not always consistent: heterogeneous samples and different kinds of physical activity could partially explain these discrepancies. Sports trainers and coaches should take into account the influence of both the time of day and chronotype effect when scheduling training sessions into specific time periods.

/pdf/chronotype-physical-activity-and-sport-performance-a-2o7fedei1w.pdf

Chronotype, Physical Activity, and Sport Performance: A Systematic Review

Purpose
The aim of this study was to compare morning and evening time-trial performance, RPE and mood state of trained swimmers, taking into account chronotype, habitual training time-of-day and PERIOD3 (PER3) variable number tandem repeat genotype.

Factors to consider when assessing diurnal variation in sports performance: the influence of chronotype and habitual training time-of-day

Whether you are a morning lark or a night owl has proven to be a key contributor in the timing of peak athletic performance. Recent evidence suggests that accounting for these differences, known as one’s chronotype, results in significantly different diurnal performance profiles. However, there is limited research investigating multiple measures of performance simultaneously over the course of a socially constrained day. This study aimed to investigate the impact of chronotype on indices of cognitive and physical performance at different times of day in healthy volunteers. We recruited 56 healthy individuals categorised as early (ECT, n = 25) or late (LCT, n = 31) chronotypes using the Munich ChronoType Questionnaire, circadian phase markers and objective actigraphy. Measures of cognitive and physical performance, along with self-reported daytime sleepiness, were taken at multiple times of day (14:00 h, 20:00 h and 08:00 h the following morning). Here, we find significantly different diurnal variation profiles between ECTs and LCTs, for daytime sleepiness, psychomotor vigilance, executive function and isometric grip strength. LCTs were significantly impaired in all measures in the morning compared to ECTs. Our results provide evidence to support the notion that ‘night owls’ are compromised earlier in the day. We offer new insight into how differences in habitual sleep patterns and circadian rhythms impact cognitive and physical measures of performance. These findings may have implications for the sports world, e.g. athletes, coaches and teams, who are constantly looking for ways to minimise performance deficits and maximise performance gains.

/pdf/the-effects-of-time-of-day-and-chronotype-on-cognitive-and-4csd188h9k.pdf

The effects of time of day and chronotype on cognitive and physical performance in healthy volunteers

A link between diurnal preference and a variable number tandem-repeat (VNTR) polymorphism in the PERIOD3 gene (PER3) has been demonstrated: the longer PER3 5 and shorter PER3 4 alleles with preferences for mornings and evenings, respectively As many competitive events in South Africa for individual athletes are scheduled for the early mornings, we hypothesized that this might favor those athletes with a preference for morning activities Selfselected white, male cyclists (CYC, n = 125), runners (RUN, n = 120) and Ironman triathletes (IM, n = 287) of European descent were compared with a control population of active, non-competitive individuals (CON, n = 96) The chronotypes of all CYC, RUN and CON participants and a sub-sample of the IM group (n = 49) were assessed using the Horne–Ostberg Morningness–Eveningness Questionnaire, and the PER3 VNTR genotype for each participant was determined The athlete groups contained more morning-type individuals than the CON group (CYC: 72%, n = 90; RUN: 67%, n = 80; IM: 59%, n = 29; CON: 41%, n = 39; p < 001) The prevalence of the PER3 5 allele was greater in the athlete groups (CYC: 61%, n = 152; RUN: 58%, n = 132; IM: 56%, n = 324; CON: 38%, n = 76; p < 001), and more athletes were genotyped as PER3 5/5 than CON individuals (CYC: 41%, n = 51; RUN: 23%, n = 26; IM: 28%, n = 81, CON: 9%, n = 8; p < 001) A strong relationship between chronotype and PER3 VNTR genotype was observed (p < 001) Finally, the time of day at which the athletes preferred to train was related to their chronotype (p < 001) This is the first study of its kind in a South African sporting population, and the results have not yet been replicated These data suggest that white males of European descent participating in individual endurance sports in South Africa are more likely to be morning types Furthermore, the PER3 VNTR may be one of the factors contributing to this observation (Author correspondence: lauraroden@uctacza)

Kim J. Stephenson

Papers

Factors to consider when assessing diurnal variation in sports performance: the influence of chronotype and habitual training time-of-day

Chronotype and PERIOD3 variable number tandem repeat polymorphism in individual sports athletes.