Morning exercise 'hits immune system'

This is because the natural cycle of their immune system, coupled with the effects of the exercise, reduces their defences against bacteria and viruses.

Dr Lygeri Dimitriou, a researcher at Brunel University in Middlesex, found levels of body chemicals strongly associated with the immune system were different depending on the time of day.

Adding vigorous training into the mixture at certain times could suppress immunity.

She looked at 14 male swimmers, with an average age of 18, who regularly trained at 6am - a common time for elite swimmers to be in the pool.

She tested levels of the stress hormone cortisol, which is known to suppress immune activity, and another chemical called IgA.

This second chemical can be found in saliva and nasal secretions and is the "first line of defence" against bacterial or viral attack, hopefully destroying airborne infections before they can take hold.

Dr Dimitriou found that cortisol levels were highest in the early morning, regardless of whether or not the person was taking exercise.

The workout only served to increase cortisol.

'Vulnerable'

She also found the rate the IgA was secreted was naturally lower in the morning than in the evening.

The overall result is that athletes who train in the morning could be vulnerable, she says.

This effect could be heightened if they are feeling under the weather in the first place or just returning from illness or injury.

Athletes training at high altitude would also be threatened, as this is known to have an immunosuppressive effect.

She told BBC News Online: "I would advise athletes to avoid training in the morning under these circumstances, but things won't be so bad for someone who is just going for a morning jog."

Her research is published in the British Journal of Sports Medicine.