My brain's tired! Scientists looking at seratonin find you may well be right

A woman yawns, posed by model (Getty Creative)

Physical exhaustion can occur when the brain – as well as the muscles – grows tired according to a study that sheds fresh light on the role played by the mind in determining endurance levels.

Scientists have found that a key neurotransmitter in the brain, which controls signalling between nerve cells, can determine whether someone feels exhausted following physical exercise or after taking anti-depressant drugs such as Prozac.

Although levels of serotonin rise during exercise, which provides a psychological boost and “feel-good” factor, it can also result in a widespread central fatigue that ultimately leads to someone feeling exhausted and unable to carry on, scientists found.

Researchers led by Professor Jean-Francois Perrier of the University of Copenhagen found that while serotonin helps to keep people going during the early stage of vigorous exercise, a build-up of the neurotransmitter in the brain can have the opposite effect by causing “central fatigue” of the nervous system even when the muscles are still able to carry on.

“We can now see it is actually a surplus of serotonin that triggers a braking mechanism in the brain. In other words, serotonin functions as an accelerator but also as a brake when the strain becomes excessive,” said Professor Perrier, whose study is published in the Proceedings of the National Academy of Sciences.

“The discovery brings us a step closer to finding ways of controlling serotonin. In other words, whether it will have an activating effect or trigger central fatigue. It is all about selectively activating the receptors which serotonin attaches to,” Professor Perrier said.

“For selective serotonin re-uptake inhibitors [eg Prozac] we can possibly help explain why those who take the drugs often feel more tired and also become slightly clumsier than other people. What we now know can help us develop better drugs,” he said.