Chronotype, sports and public health

‘‘Chronotype’’, addressed in the Journal in 1997 (Tassi & Pins, 1997) and in 38 publications since 2014 (Broms et al., 2014; Erren & Morfeld, 2014; Laborde et al., 2015; Vitale et al., 2015) alone, is a concept which should be diligently explored. Ehret (1974; p. 74) introduced the term to describe ‘‘the temporal phenotype of an organism’’. As a biological trait, it holds information when individuals tend to be awake and asleep, i.e. when biological days and nights are timed. It thus captures information on ‘‘circadian highs and lows’’ of physiology, endocrinology, metabolism and behaviour; it is genetically co-determined (Toh et al., 2001) and sex-, ageand light-dependent (Roenneberg et al., 2003, 2007). With multiple determinants and many implications, understanding what affects chronotypes and how chronotypes may influence sleep–wake patterns – and their consequences – over time could be relevant for public health. Key insights into chronotype as an individual’s phase of entrainment of rhythms of rest and excess (Foster & Wulff, 2005) could come from sports studies. In this vein, Henst et al. (2015) investigated relationships between chronotype, on the one hand, and training times, race times and marathon performances, on the other, in Caucasian males. Interestingly, among marathon runners (RUNSA and RUNNL) and controls who exercised but did not participate in competitive sports (CONSA and CONNL), morning-orientation was overand evening-types were under-represented. Therefore, could there be a general tendency towards ‘‘higher morningness’’ in competitively and leisurely active athletes? Empirical data in addition to the ones mentioned by Henst and colleagues, i.e. by Kunorozva et al. (2012) and by Lastella et al. (2010), are compatible with this notion: Silva et al. (2012) observed that in a Paralympic track and a field athletes group 21 out of 26 individuals were either morning or extreme morning types [assessed via the MEQ (Horne & Ostberg, 1976)]; Samuels (2008) found 35.6% and 42.9% morning types in a Canadian sport school (n1⁄4 45) and in Canadian bobsleigh athletes (n1⁄4 21; assessed via the Athlete Morningness/Eveningness Scale; AMES-Questionnaire), respectively. Hypothetically, a higher prevalence of morning types in some sports might be explained in the following way: If loads of training are shouldered by including early mornings as activity time windows, such stress and strain would primarily concern those who train; if, however, training sessions are shifted to later time windows of the day, including evenings, the sport of an individual could affect other lives – families, partners, friends – more directly. Compared with late chronotypes, morning types should find it easy to choose the former and to avoid the latter scenario. Dual, i.e. performance and social, benefits would thus favour a selection of morning types in some sports explaining the observed chronotype distributions among athletes pointed to above. However, beyond plausible social determinants of when people engage in sports, let us focus on how chronotype may factually be associated with training times, race times and performances. Critical questions include ‘‘what are the causal relationships? ’’ or ‘‘which is the cart and which the horse? ’’ (Hill, 1965). The authors (Henst et al., 2015) acknowledge the difficulty of understanding the direction of such relationships: ‘‘. those who were more morning orientated tended to train on more days and for more hours each week. This correlation may either suggest that people who are orientated towards morningness may choose to be more physically active, or that the chronotype of people who are more physically active may shift over time so that