Soccer is one of the most played sports in the world, where players need technical, tactical, and physical skills to succeed. Cardiorespiratory fitness and therefore maximal oxygen uptake (V[O.sub.2max]), is one of the most important parameters affecting soccer performance with a clear relationship with the distance covered, work intensity, number of sprints and involvements with the ball during a match (Helgerud et al., 2001). Helgerud et al. (2007) clearly demonstrated that a high-intensity interval training (HIIT) is significantly more effective than performing the same total work at either lactate threshold or at 70% of maximal heart rate ([HR.sub.max]), in improving V[O.sub.2max]. However, this type of protocol is physically demanding and it does not allow for complete recovery after training. In fact, according to Seiler et al., (2007) elite soccer athletes during their training session perform very high training loads, submitting the body to a very high stress which could induce adaptive effects which are cumulative and incomplete recovery from them can lead to non-functional overreaching or overtraining. For this reason, the day-today distribution of training intensity may be a crucial variable to effectively balance positively and negative stress effects so that performance development is achieved without stagnation or overtraining (Uusitalo, 2001).
One of the most non-invasive reliable methods to assess cardiovascular recovery after training is to evaluate the time course of the heart rate variability (HRV), which is the natural fluctuation of HR in time which reflects the neural control of the heart via sympathetic and parasympathetic innervation on the sinus node (Aubert et al., 2003). The quantification of HRV is one of the key aspects of this stress response, consisting in activation of the sympathetic arm of the autonomic nervous system (ANS) and a shift in autonomic balance. Generally, sympathetic predominance has been observed 1 hour after the cessation of exercise and pre-exercise levels appeared to be regained only 24 hours after the exercise bout (Bernardi et al. 1997; Furlan et al. 1993). In addition, a rebound of the parasympathetic activity was also found two days after prolonged exercise (Hautala et al. 2001). On the other hand, the exercise performed in the aforementioned studies for their length and environmental conditions differ from those that athletes would routinely perform in a training program. Indeed, changes in HRV were reported after 30 min of maximal exercise leading to exhaustion (Furlan et al. 1993), 46 km of running at a mean altitude of 2,500 m (Bernardi et al. 1997), and 75 km of cross-country skiing (Hautala et al. 2001). Regarding HIIT Al Haddad et al. (2009) found that a single supramaximal exercise bout, lasting no more than 12 min, can perturb the ANS for about 36 hours, confirming that there is also a strong influence of exercise intensity on short- and long-term post exercise HRV recovery. For what concern team sport activities, Flatt et al. (2017) observed in a group of female collegiate soccer players that an intense training was associated with a reduced parasympathetic activity which was associated with a higher training load due to an increased training stress. Moreover, Esco et al. (2016) found an association between the change in weekly parasympathetic activity and the adaptation of aerobic power following an off-season program suggestion that this could be an efficient method to evaluate the aerobic adaptation in female soccer players.
Another important variable which may determine different autonomic responses during recovery from a demanding training bout, are circadian rhythms. In fact, individual differences in these chronobiological rhythms can be summarized under the concept of Morningness/Eveningness, also termed chronotype. These rhythms influence our daily behavior. People typically, based on one's innate circadian rhythm, display preferences for activity at different time of day (Montaruli et al., 2017; Roveda et al., 2017). These individual differences in the phase of biological and behavioral patterns are genetically based and controlled by an endogenous circadian pacemaker, although other factors such as social and cultural influences might contribute to determining differences in behavioral rhythms. This circadian phenotype differs among individuals and may be classified as: Morningtype (M-type), Evening-type (E-types) and Neither-type (N-type) (Natale and Cicogna, 2002). Research often focused on the differences between M- and E-type individuals. M-types wake up early and perform mentally and physically at their best in the morning hours, compared to E-types who plan their daily activities for the afternoon or evening and prefer to stay out late (Bonato et al., 2017). Recent studies clearly showed large differences among chronotypes for several physiological variables: sleep behaviour (Vitale et al., 2015; 2017), hormones secretions (Bailey and Heitkemper, 2001), physical performance (Rossi et al., 2015; Vitale et al., 2013), and HRV (Roeser et al., 2012).
Regarding HRV, previous findings support the assumption that E-types presents a significantly higher HR and systolic blood pressure but lower HRV than M-types both at baseline and during mental stress conditions (Roeser et al., 2012), demonstrating that stress induced in the evening had a significantly higher impact on cardiovascular responses than stress induced in the morning independent of chronotype. On the other hand, Roeser et al., (2012) induced stress using an arithmetical task. Proven that supramaximal intermittent exercise perturbed the ANS for more than 24 hours (Al Haddad et al., 2009), we hypothesized that there could be differences in post exercise autonomic control according to the time of the day at which this training is performed and chronotype. Therefore, the purpose of this study was to evaluate the influence of the time of the day (8.00 a.m. vs 8.00 p.m.) and chronotype on pre- and post-exercise autonomic cardiac control in soccer players in relation to an acute session of HIIT.
Participants were recruited among Bachelor's students of the School of Sport Science of Universita degli Studi di Milano, Milan, Italy during the academic year 2015-2016. Inclusion criteria for subject's participation to the study were: age [greater than or equal to] 18 years; male; being soccer players; 6 hours of training a week and with a morning or evening chronotype scores (see "assessment of circadian typology"). Exclusion criteria were smoking, use of medications and any other medical condition contraindicating physical exercise. Twenty-four collegiate male soccer players (12 M-Types and 12 E-Types) were therefore deemed eligible and enrolled in the study. Before entering the study, the participants were fully informed about the study aims and procedures, and written informed consent was obtained before testing. The study protocol was approved by the Institutional Ethics Review Committee of the Universita degli Studi di Milano (approved on 12/10/15, prot. N. 52/15) in accordance with current national and international laws and regulations governing the use of human subjects (Declaration of Helsinki II). This trial was registered at Australian New Zealand Clinical Trials Registry (ACTRN12617000432314). After a baseline assessment consisted of an anthropometric evaluation, subjects underwent a Yo-Yo Intermittent Recovery Test Level 1 and then they were randomly assigned in a 1:1 ratio according to their chronotype to either morning training (Group A: n = 12; age 23 [+ or -] 3 years; height 1.75 [+ or -] 0.07 m; body mass 73 [+ or -] 10 kg, weekly training volume 8 [+ or -] 2 hours) that started performing the high intensity interval training protocol at 8.00 am or evening training (Group B: n = 12; age 21 [+ or -] 3 years; height 1.76 [+ or -] 0.05 m; body mass 75 [+ or -] 11 kg, weekly training volume 8 [+ or -] 3 hours) that started performing the high intensity interval training protocol at 8.00 pm. Both groups were blinded about the aim of the study.
This was a randomized crossover study which was carried out in spring, between March and April 2016, over a period of four weeks and the experimental design consisted of the following: Group A performed the morning training session at 08.00 am while Group B performed the evening training session at 08.00 pm; after a recovery period of 7 days in which subjects maintained their habitual lifestyle without performing physical training, Group A performed in the evening while Group B performed the training session in the morning. One participant in the Group B dropped out because he did not perform the second training session (Figure 1). In each test session, the measurement of HRV (see "heart rate variability assessment") was performed at REST, before (T0), and 12 (T12) and 24 (T24) hours after the training session. All subjects were familiarized with all testing procedures.
Assessment of subject's circadian typology
Participants' circadian typology was assessed by...