The time of day that an Olympic swimming race takes place may affect athletes’ performance and could mean the difference between gold, silver and bronze, according to a study published in Scientific Reports.

 Countries selected to host the Olympics may adjust race times to accommodate broadcast across continents, requiring athletes to compete at different, sometimes unusual, times throughout the day. However, the potential impact of race start times on performance has not been widely studied. Renske Lok and colleagues from the University of Groningen and Chrono@Work analysed data on the race times and results of 144 Olympic swim finalists collected during the Games in Athens (2004), Beijing (2008), London (2012) and Rio de Janeiro (2016). The authors found that physical performance was significantly affected by time of day. The fastest swim times were recorded in the late afternoon, around 5pm, and found to be a 0.32% improvement relative to races scheduled for 8am – a difference of several seconds. The time-of-day effect exceeded the difference in swim times between first and second place in 40% of the races and second or third place in 64% of the races; in finals, this may mean the difference between getting gold, silver or bronze medal, or no medal at all. The findings suggest that despite elaborate training schedules ranging from morning to evening hours, the time of day a race is scheduled may still affect athletes’ performance. Athletes may consider adjusting their internal body clock to better match their peak performance for events with unusual start times.