Eyes feeling tired? It could be due to the strain of working out, a Kiwi scientist's team has discovered.

But there's also some good news -- the researchers, led by Auckland University's Dr Nicholas Gant, have found a cup of coffee can help.

Scientists from four countries tested cyclists in a lab at Auckland University for three hours. At the end of the work-out, the researchers tested the cyclists using specialised eye-tracking cameras.

The results, published in Nature Scientific Reports, show for the first time that rapid eye movements slow down when we are fatigued.

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"It's remarkable that tiring the legs also slows the eyes," Dr Gant said.

Strenuous exercise appeared to cause an imbalance in neurochemicals, which spread across the brain's control systems, he said.

"These results are important because our eyes must move quickly to capture new information," Dr Gant said.

"But there's hope for coffee drinkers because this visual impairment can be prevented by consuming caffeine."

Dr Gant said even a modest dose of caffeine could restore chemical balance, helping signals from the brain reach the eyes.

"The amount of caffeine we gave during exercise was the equivalent of two cups of coffee. We saw no effect with a decaffeinated placebo drink." he said.

"Interestingly, the areas of the brain that process visual information are robust to fatigue. It's the pathways that control eye movements that seem to be our weakest link".

The team is now investigating the effects that psychiatric drugs -- used to treat patients with abnormal levels of these neurotransmitters -- have on this phenomenon.