In an article titled “Morning Workouts are the Secret Pride of Swimmers,” author and collegiate swimmer Sarah Lloyd wrote “morning workouts are simultaneously the bane of and the reason for a swimmer’s existence (Lloyd, 2020).” Many coaches would be reluctant to alter workout times because of logistical difficulties (see below); not wanting to “pamper” athletes; and the “if it’s not broken, don’t fix it” rationale based on the storied history of swimmers who thrived on morning training (Lloyd, 2020).

The discipline required to, for example, regularly dive into cool water at 6 a.m., and other intangible factors such as the pride of doing so and the satisfaction associated with starting one’s day with a hard workout, could be irreplaceable. This is an empirical question that is worthy of investigation because the system of early morning training could indeed be “broken” for many young athletes. There could be Michael Phelps-level talent who would excel with later workout times but either never considered swimming because of the schedule; quit the sport; or failed to reach their potential because of an inability to shift their stubborn body clocks.

An analogy worth considering is a study of soldiers participating in basic combat training (Miller et al., 2012), another setting with a longstanding tradition of early morning physical training in young adults (Crowley et al., 2012). The study found that assigning half the soldiers to a later sleep schedule (11 p.m.–7 a.m.) compared with the control soldiers’ schedule (8:30 p.m. to 4:30 a.m.) resulted in significantly less fatigue, better marksmanship, and less total mood disturbance (Miller et al., 2012), which has been associated with better athletic performance in other studies (Morgan et al., 1987).

In summary, tradition and coaches’ attitudes are significant barriers to enacting the proposed changes in training schedule of two-a-day athletes. Nonetheless, definitive evidence supporting the benefits of such as change could persuade coaches to do this.

One approach would be to shift the start of the 6 a.m. workout to 9 or 10 p.m. for the entire team, which could be beneficial for most of the team. However, this could introduce safety challenges for workouts for some sports, such as running and cycling. There could also be safety concerns regarding commuting to and from the practice facilities, but perhaps not much different than those faced with 6 a.m. workouts for some of the year. The logistics of opening the workout facilities in the evening would be another challenge.

One concern might be that evening could disturb sleep, which would partly defeat the purpose of the proposed change. Nighttime athletic competition has resulted in disturbed sleep in some studies (Nédélec et al., 2019), but competition is associated with a far higher level of psychophysiological arousal than workouts. The available evidence indicates that even vigorous evening exercise ending 2-4 before bedtime does not disturb sleep for most individuals (Youngstedt et al., 1999; Buman et al., 2014; Vitale et al., 2017; Frimpong et al., 2001; Youngstedt et al., 2021), and with experience, it is possible that the minority of individuals with disturbed sleep after evening exercise can adapt to evening exercise without experiencing disturbed sleep.

Another option would be to shift the early morning workout to either the evening or later in the morning for some of the team, and keep the early morning workout for some of the team. However, this would introduce logistical challenges involving more hours of facility operation and more demands on the coaches’ time. Separating athletes could also be detrimental to team cohesion and the benefits of training with athletes of similar levels of performance.

In summary, logistical and safety issues are also significant barriers to shifting early morning workout times to later in the day. Whether these barriers can be overcome could vary for different teams, and might depend on the strength of research indicating advantages to making such changes.

One of the rationales for morning swim training is that it will translate to better morning performance in preliminary trials, which are generally in the morning (e.g., 9 or 10 a.m. at the NCAA championship meet), and good performance in the trials is necessary to advance to the finals. Empirical support for this rationale is mixed.

There is a circadian rhythm in athletic performance with peak performance in the late afternoon to early evening and nadir of performance in the early morning (Youngstedt et al., 2019). However, Rae et al. found that swimmers who regularly trained only in the morning (average of 6 a.m.) swam faster in time trials at 6:30 a.m. compared with 6:30 p.m. (Rae et al., 2015). Nonetheless, this result could be partly explained by the fact that most of the morning swimmers were morning larks with a natural tendency to better morning performance.

Experimental studies have indicated that superior anaerobic exercise performance in the afternoon vs. morning can be significantly attenuated by anaerobic exercise training in the morning (6 a.m.) (Souissi et al., 2002; Chtourou and Souissi, 2012), and other research has indicated a greater improvement in anaerobic threshold in the morning and afternoon following training in the morning and afternoon, respectively (Hill et al., 1989). These results are consistent with compelling evidence that the time of day of exercise can shift the skeletal muscle clock (Martin and Esser, 2022) which could impact performance.

However, there are limitations of these data that make generalization to competition in elite athletes dubious. First, such evidence has not been shown in elite athletes. On the contrary, in a study of mostly collegiate swimmers, 200 m swim performance averaged almost 6 s slower at 3 a.m. and 6 a.m. compared with 8 p.m. and 11 p.m., despite regular morning training (Kline et al., 2007). Second, specificity of training effects have been shown for training at one time per day, and not for training twice per day. Third, specificity has been shown for performance measured within 30 min of training time. It is unclear, whether there is a similar advantage of training at 6 a.m. for performance at 9 or 10 a.m.

Nonetheless, the assumption persists that early morning workouts confer a competitive advantage for late morning competition Further research might be needed to confirm or dispel this assumption before coaches and athletes might be willing to forgo early morning workouts. Should more definitive evidence arise supporting this assumption, a subsequent important question is how long one must train in the morning to confer an advantage to morning performance. A study by Soussi et al. found that only 2 weeks of training twice per week in at 7–8 a.m. conferred improvement in Wingate and peak knee extension torque assessed at 7–8 a.m. (Souissi et al., 2002), and evidence indicates that a single bout of exercise can shift muscle clock gene expression in mice (Kemler et al., 2020). Thus, conceivably, athletes could experience a season of sufficient sleep, and benefit from a short period of early morning training close to major competitions.