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Usain Bolt smashed the 100-metre sprint record at the 2009 World Championships in Athletics in Berlin in a floodlit stadium under an inky night sky.

This was no coincidence: when it comes to sporting prowess, timing matters. For activities relying on sheer muscle power and endurance, evening or late afternoon are when most world records have been set, probably due to assistance from peaks in the daily rhythms of a number of the body’s key physiological outputs.

But what about everyday exercisers? Is there an optimal time of day, or month, to get the most out of workouts and reduce the risk of injury?

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“Whichever aspect of sport you look at – whether it’s sports medicine or exercise response – time of day matters,” says at the University of Manchester, UK. Later afternoon or evening is when our body temperature peaks, resulting in faster metabolic reactions and nerve signal transmission compared with the early morning. Connective tissue is also more flexible in the afternoon, while our reserves of glycogen – the biochemical energy source our muscles draw on during moderate to intense exercise – have had time to be replenished.

Circadian rhythms

, which could be relevant for exercise: testosterone secretion peaks at around 9am; coordination tends to be best at around 2.30pm; reaction times are fastest at around 3.30pm; cardiovascular efficiency, muscle and grip strength peak at 5 to 5.30pm.

“Circadian rhythms exist in almost all cells of the body and regulate key processes relating to exercise and metabolism,” says Meng. “Depending on what time you exercise, your muscle, fat and other cells will also be in a different state, and they will respond to exercise differently.”

Indeed, a recent study by at Stanford University in California and her colleagues found that, on average, if they compete in the evening rather than in the morning. “In 40 per cent of [swimming] races, the time-of-day effect is bigger than the difference between finishing first or second,” they wrote.

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For sports involving more technical skills, such as tennis or soccer, peak performance tends to arrive a little earlier – possibly because our cognitive abilities usually peak in the late morning or early afternoon. Soccer players at around 4pm; , but more accurate in the morning. Of course, these timings are based on averages – in reality, “larks” who tend to wake early and be more active in the mornings will be at their best earlier, while “owls”, who naturally come to life in the evenings, will peak later.

What are the implications of all this for how the rest of us time our exercise? Last year, at the University of Basel in Switzerland and her colleagues and found little to support or refute the idea that training at a specific time leads to better performance or improved health outcomes.

They did, however, find some evidence to support training at the same time of day as a race or competition to improve physical performance at this time. In other words, morning training improves morning performance more than evening training does, and vice versa. However, given that the studies only included young male participants, it remains to be seen whether such conclusions apply to the general population, the authors said.

Monthly cycles

Women may have a further layer of complexity to consider. In recent years, several women’s soccer teams, including Chelsea FC Women, have started tailoring their players’ training programmes around their menstrual cycles, claiming that doing so boosts their performance and reduces their risk of injury.

“The theory is that when oestrogen is high and progesterone is low… that’s an anabolic environment; it’s a good [time] to work hard,” says at McMaster University in Canada. Yet when he and his colleagues recently for an influence of menstrual cycle phase on exercise performance, they found it was “remarkably thin”. “The evidence that we do have suggests that there’s no merit to it,” says Phillips.

Even so, he doesn’t dismiss recording symptoms and using them as a guide to scheduling training. “I know some women are genuinely adversely affected at certain phases of their cycle with menstrual-related symptoms: cramps, backaches, a lack of motivation, fatigue, etc. And for women athletes that do experience symptoms, trying to manage them and making their coach aware of them is a great thing,” he says.

“But as for a blanket, ‘this-is-the-way-you-do-things’-type approach, there’s no consistent pattern to performance when it’s studied systematically, and we know gold medals and world records have been set at different [menstrual] phases, and on and off the contraceptive pill.”

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Meng believes there may be yet other factors to consider. Generally, he advocates exercising in the morning – especially outdoors – as this exposes people to bright light, which helps to synchronise our biological clocks with the time of day. Our bodies work best when the clocks in all our cells and tissues are aligned with one another, and with the time of day.

And even if there isn’t a “best” time of day to work out, there may be a time to avoid exercise. Recent research by Meng and his colleagues has suggested that a key mechanism for keeping the biological clocks in our bones and joints synchronised with those in other tissues is exercise, and that if mice are encouraged to exercise when they would usually be sleeping, this causes their skeletal clocks to – a phenomenon Meng has named “skeletal jet lag”.

Though the implications for injury and physical performance in humans are unclear, further experiments in mice suggest that consistently exercising during their equivalent of night-time leads to the activation of genes that are associated with osteoarthritis. “We suspect that if you did this over a long period of time, it could be really detrimental,” says Meng.