by Renske Lok, Department of Psychiatry and Behavioral Sciences, Stanford University
Renske Lok PhD is a student board member of the Society for Light Treatment and Biological Rhythms, and a postdoctoral researcher at Stanford University. Her research focuses on effects of light on human alertness, thermoregulation, sleep and physical performance.
When we watch sports games or races, such as at the Olympics, it becomes clear that the difference between finishing first or second can come down to a matter of milliseconds. With such miniscule differences, every aspect of physical performance has to be optimized. The circadian system — our biological clock — regulates body processes with a rhythm of approximately 24 hours. Circadian rhythms influence many aspects of performance, ranging from physiological effects — such as limb movement speed, muscle strength, and muscle flexibility — to motivational, perceptual and cognitive factors.
Recently, my collaborators at the University of Groningen in the Netherlands and I assessed the circadian rhythm in physical performance in Olympic athletes: their worst performance was in the early morning around 5 a.m., while they were best in the late afternoon around 5 p.m. A straightforward recommendation, then, might be that races should be held in the late afternoon to ensure that all team members can perform at their circadian peak. However, the timing of the circadian clock differs markedly between individuals. This concept is called “chronotype,” the behavioral expression of the underlying circadian rhythm. Chronotypes can be divided into three main categories: morning type (with an early sleep period, the “larks”), intermediate types, and evening types (with a delayed sleep period, the “owls”). The peak time in optimal performance varies with chronotype. For example, extreme owls are more likely to perform better in the late afternoon compared to extreme larks, who tend to peak earlier in the afternoon.
To optimize physical performance during an important race or game, individuals need to align their circadian rhythms (internal clock time) with the timing of a race or game (external clock time). The circadian clock is not immutable — it can be shifted with light administration or minimization at appropriate times. An athlete’s circadian clock can be shifted such that if a race is scheduled in the early morning (external clock time), the body (internal clock time) can behave as if the race were in the late afternoon. This adjustment is called “phase advancing.” Minimization of evening light, and maximization of morning light exposure will cause a shift of circadian rhythms (and the sleep-wake cycle) earlier. Alternatively, if the race occurs in the late evening, the individual may choose to phase delay their circadian rhythms and sleep by opposite rules, minimizing morning light exposure and maximizing evening light exposure.
Therefore, by adjusting internal clock time to match a different external clock time than is normal for you, you can optimize your physical performance for critical events, like a morning race. Plan to begin shifting the light pattern days ahead of the event to reach a stable, optimized performance level, and meet the challenge to the best of your ability.