Scientists agree: this is the biological change soccer players experience on game day

Elite athletes often talk about feeling different on game day, but new research suggests those changes begin long before they step onto the field.

A study led by researchers at the International University of La Rioja (UNIR) in Spain has found that professional athletes experience a dramatic surge in cortisol – commonly known as the body’s primary stress hormone – as soon as they wake up on competition days. The findings were published in the journal Psychoneuroendocrinology.

The body starts preparing hours before kickoff

The researchers found that athletes’ cortisol levels increased by 82.1% immediately after waking on game days, compared with just 24.7% on rest days. In other words, the normal biological response was more than three times stronger when competition lay ahead.

This phenomenon is known as the Cortisol Awakening Response (CAR). According to the researchers, it is a crucial neuroendocrine mechanism that helps mobilize energy, sharpen alertness and prepare the body’s metabolic and nervous systems before competition begins.

To reach their conclusions, the team analyzed saliva samples from 190 elite athletes – 58 women and 132 men – across seven different sports, including professional soccer players competing in Europe’s top leagues.

The findings support what’s known as the “anticipation hypothesis,” suggesting the body begins preparing for an expected challenge almost as soon as a person wakes up.

“The cortisol awakening response could be an anticipatory activation – an urgent, acute mobilization linked to our subjective assessment of the challenges we will face that day,” said Manuel Jiménez, a UNIR researcher involved in the study.

Jiménez believes the response may have deep evolutionary roots.

“It was probably a biological advantage to be alert and ready to respond to threats immediately after waking, coinciding with increasing daylight,” he explained. He added that an athlete whose cortisol response matches the demands of the upcoming competition could enjoy an important competitive advantage.

A new tool for coaches and medical staff?

The researchers also believe the cortisol awakening response could become a valuable tool for coaches and sports scientists.

Not every athlete responds the same way. The study found 47.3% individual variability in cortisol responses, suggesting each player has a unique biological “activation clock.”

Monitoring these patterns could help identify athletes who are underactivated before competition or those experiencing excessive stress. Either extreme could affect performance, allowing coaching staff to intervene before games.

The researchers also say the findings may have implications for injury prevention. Jiménez’s previous work has linked elevated stress with a greater likelihood of muscle injuries in professional soccer.

“If a player is exposed to excessive stress or experiences disrupted sleep – something common during training camps and major tournaments – the endocrine system’s ability to respond can be affected,” Jiménez said. “That not only influences performance but is also directly associated with a higher risk of muscle injuries.”

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