Physical exercise can tire the eyes as well as the muscles, but a cup of coffee can revive them, the New Zealand leader of an international study said Thursday.
A team of scientists from four countries found the first scientific evidence that rapid eye movements slow down when the body is fatigued.
"These results are important because our eyes must move quickly to capture new information," said study leader Dr Nicholas Gant, of the University of Auckland.
"But there's hope for coffee drinkers because this visual impairment can be prevented by consuming caffeine," he said in a statement.
In the study, cyclists exercised in a laboratory for three hours, after which their brain's control of the visual system was tested using specialized eye-tracking cameras.
"It's remarkable that tiring the legs also slows the eyes," said Gant.
"This might well be the reason the tired cyclist never saw that bus coming."
An imbalance in neurochemicals caused by strenuous exercise appeared to spread across the brain's control systems.
But 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," said Gant.
"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 was investigating the effects that psychiatric drugs -- used to treat patients with abnormal levels of these neurotransmitters -- had on the phenomenon.
