The coronavirus disease 2019 (COVID-19) pandemic and associated lockdowns transiently changed soccer (football) globally. Given that team performance in soccer is multifaceted, it is unsurprising that almost all investigated components were affected by lockdowns. Indeed, government-mandated restrictive measures (eg, quarantine, curfew, or stay-at-home orders; termed simply "lockdown" from here on) compromised (or prevented) access to sporting facilities, resulting in lower loads (volume, frequency, and intensity) and altered modalities of training, often holistically manifesting as insufficient or nonspecific training stimuli. Moreover, lower mood state and emotional intelligence, associated with lower motivation to train, were also reported. Lockdown modified and disrupted lifestyle behaviors (eg, eating, training and sleeping) and external zeitgebers (eg, work schedule, meal timing, sunlight exposure, team training) related to circadian regulation/rhythms of athletes, contributing to lower sleep quality and increased depressive symptoms, which likely affected physical and mental health of athletes during lockdown. Interestingly, recent evidence suggest that some subgroups of athletes (eg, male vs female, elite vs nonelite, younger vs older) were more susceptible to lockdown-mediated effects regarding their sleep and training loads than others. In addition, Ramadan (2020) occurred at the same time as the early 2020 lockdown, which could further disrupt Muslim players` sleep and training behaviors. Specific to soccer, the lockdown increased negative emotions and depressive symptoms in professional players, especially those who were infected by the virus. Reduced physical performance in professional players has been reported at the resumption of competition with some reporting an increase in injuries. Although the effect of lockdown on athletes` sleep quality is now well documented specific data about soccer players is scarce. Of particular interest, given professional soccer was one of, if not the first, global sport to return to training and competition. To the authors` knowledge, limited information on the effect of lockdown on sleep and training behaviors worldwide are available, with the only existing study in soccer reporting local data. Given the above, it appears prudent to explore the within soccer effects of the early 2020 COVID-19 lockdown upon the sleep and training loads of an international sample of players (including relevant subgroup comparisons). It has been hypothesized that the COVID-19-induced lockdown would reduce sleep quality, increase insomnia severity, and affect training behaviors in this sample of international soccer players. Furthermore, it was expected that subgroups of female, elite, Muslim, and older players would be more affected by the lockdown.
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