Morning exercise can make you sick

Early morning joggers, swimmers and fitness fanatics who use the gym before breakfast are putting themselves at greater risk of infections than evening exercisers, scientists claim today.

A study by university researchers reported that while a few early miles on the treadmill or in the pool may be a wake-up call to the body, it could be opening the way for all kinds of bugs. While a dip in the pool or a run to work might get other senses working, the body's immune system needed a lie-in and was not ready to cope with the physical exertion.

And serious athletes, as well as part-time weight-watchers, should think twice about the dawn run.

Scientists at Brunel University in Middlesex tested 14 British competition swimmers, putting them through their paces at both six in the morning and six in the evening, on separate days.

The scientists took saliva samples before and after the exercise sessions, and told the young swimmers not to drink, smoke or have sex for 12 hours beforehand.

Tests showed that levels of cortisol, a stress hormone which can suppress the immune system, were slightly lower in the evenings. But levels of the chemical IgA, which helps protect the body against infections, were down dramatically in the morning, which meant the swimmers were running the risk of picking up infections.

This would apply to any athlete, professional or amateur, who exercised early in the morning, said the scientists - and it would not matter if they were fit or fat.

The biggest factor involved in the changes was the human body clock - what scientists call "circadian rhythms". The body was simply not awake enough to handle exercising at that time in the morning, and it took a few more hours until the immune system was fully up and running. The report said: "The best time of day to train is in the evening, when baseline levels of cortisol and those produced after exercise are both lower than in the morning.

"Salivary flow rate, which also helps to protect against bugs and viruses, also peaks in the evening."

However Dr Lygeri Dimitriou, in charge of the team, said there was little full-time professional athletes could, with many needing to train three or four times a day. But, he said, athletes should still avoid early morning sessions if they were returning to training after injury or illness, were close to competition, or were stressed.