London, July 30 (Reuters)-
Early morning workouts may help to get the day off to a good start but scientists said on Tuesday they could increase susceptibility to infection.
Levels of the stress hormone cortisol which supresses the immune system are higher in athletes who train early rather than late in the day.
"The results of this study suggest that the optimal time for training-that is, the time of day with the least immunosuppresive effect-is the evening," said Dr. Lygeri Dimitriou, of Brunel University in Isleworth.
In a study reported in the British Journal of Sports Medicine, Dimitriou and his colleagues studied the effects of early morning and evening training programmes on 14 competitive swimmers.
The reaserchers measured their levels of cortisol and their IgA secretory rate, a marker of immune activity, in saliva after early and late training sessions.
Levels of cortisol were higher in the morning, both before and after the training but the IgA secretory rates were lower but not affected by the training.
The scientists said findings suggest that a persons internal clock, or circadian rhythms, have an impact on the immune system.
They suggest athletes should avoid early morning training, particularly after injury or ilness or before a big competition.
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