By Annabel Bourne for the BBC
Photo: JENS KALAENE
In a few months, the world's top athletes will gather in Paris to compete for the ultimate prize in sport - gold at the Olympic Games. For those hoping for a chance to enter the history books with a record-breaking performance, they might want to look at the hour on the clock before they settle into the starting blocks.
At least the swimmers might, according to one scientific study. Across four Olympic Games in Athens (2004), Beijing (2008), London (2012) and Rio (2016), the swim times of 144 medal-winning swimmers were found to be the fastest if they were competing in the early evening. Specifically, about 5.12pm. It is part of a growing amount of evidence suggesting that physical performance is affected by the time of day.
The phenomenon is not just found among decorated Olympians - recreational cyclists complete faster time trials in the evening. Resistance exercise is particularly susceptible to time-of-day effects, with performance nearly always peaking between 4pm and 8pm. The time of day seems to also seems to affect men and women differently when they exercise.
But what if your schedule means you only have time to exercise at 7am? There are some indications it may even be possible to adjust your peak time for athletic performance.
At the root of the differences in how our bodies perform and respond to exercise are our circadian rhythms - the body's molecular clock responsible for regulating behaviours such as sleep and appetite throughout the 24-hour period.
A central clock located in the hypothalamus of the brain responds to light exposure via signals from the optic nerve. The suprachiasmatic nucleus, as this circadian pacemaker in the hypothalamus is known, in turn sends signals to peripheral clocks in other organs, muscle tissue and fat tissue, keeping the whole body in sync. These peripheral clocks, however, can be adjusted by other cues such as when we eat or perform certain activities. The "skeletal muscle clock" responds in this way to exercise, and so can be tuned by exercising regularly at different times.
But while this can affect performance, it can also alter the effect exercise has on our health, too.
Juleen Zierath, an exercise physiologist at the Karolinska Institute in Sweden, has been researching the interaction between exercise and the circadian system. She and her colleagues found mice that exercised in the morning burn more fat. Zierath says the findings suggest that exercising at an optimal time of day could maximise the health benefits of exercise for individuals with metabolic diseases, such as type 2 diabetes and obesity.
"Everybody agrees that exercise is good, irrespective of time of day, but one can maybe fine-tune the metabolic outcomes of the exercise based on when you exercise," says Zierath.
Their findings reflect a recent study in humans that showed performing an exercise regime of resistance training, interval sprints, stretching and endurance for an hour one day a week in the morning can reduce abdominal fat and blood pressure in women. Interestingly, when women did the same exercises in the evening, it enhanced their muscular performance.
For men, evening exercise helped to lower blood pressure and stimulates the breakdown of body fat.
But research in this area is still evolving and some recent analyses of previous studies suggests the evidence is somewhat inconclusive for an advantageous time-of-day effect upon exercise performance or health benefits.
One reason for this is almost certainly the differences that exist between individuals. For example, the time of peak athletic performance differs among individuals with early chronotypes and individuals with late chronotypes, also known as morning larks and night owls.
"There are variations in the timing of our clocks," says Karyn Esser, a physiologist at the University of Florida in Gainesville in the US. "Those of us that are larks have a clock that likely runs a little bit less than 24 hours, and those of us that are owls probably have a clock that runs a little bit more than 24 hours."
But if you find your own circadian rhythms don't quite allow you to give your best performance at the times you have available, exercise may also help to "reset" your muscle clock.
A group of researchers led by Esser, found that consistent endurance running training among mice in the morning can cause the rodent's bodies to adapt to the new exercise regime. The exercise appears to shift the molecular clocks in their skeletal muscle and lung tissues to an earlier time of day.
The team's latest study, which has yet to be published in a peer-reviewed journal, found that the magnitude of adaption in performance was greater in mice trained in the morning, compared to those trained in the afternoon. After six weeks of training, both morning and afternoon mice achieved the same maximum endurance performance.
The researchers suggest that if a similar effect is found in humans, it might be possible for athletes to recalibrate their internal "muscle clocks" with the right training. There is some preliminary evidence that exercise can shift circadian rhythms in humans, making it perhaps useful for those adjusting to shift work or jet lag.
"The simple notion here is that the clocks in our muscles are actually paying attention to when we train," Esser says.
Routine appears to be key - our body adapts better to training when it is performed regularly at the same time of day.
"If you're in the general population, or even an elite athlete, and you plan to compete, you should try to have a race-day-specific training," says Zierath. "Time your training bouts so that they are consistent with the time that you're going to have to be performing or competing at your peak."
Most researchers are keen to point out, however, that exercising at any time is beneficial. But, if do you find a time that works and stick with it, your body may just adapt to give you an extra edge.
- This story was first published by the BBC.