New research from Britain now says early morning exercise may increase a person's susceptibility to infection.
The researchers studied 14 competitive male swimmers, average age 18. The swimmers swam the 400-meter crawl five times, with a minute rest between each swim. They did this on two days at 6 a.m. and 6 p.m. each day.
Samples of spit were taken from the swimmers before and after each swim in order to measure their saliva production. The saliva was measured for levels of cortisol, a stress hormone that suppresses the immune system.
The researchers also checked the swimmers' saliva for IgA secretory rate. IgA helps defend the body from infections in the nose and mouth.
The study found the swimmers' cortisol levels were higher in the morning than in the evening before exercise. The levels were significantly higher after the swimming sessions. IgA secretory rates were much lower in the morning than in the evening, but only slightly affected by the swimming.
Salivary flow rate was significantly reduced by both the morning and evening swims. The salivary flow rate was lower before the morning swims.
The authors say their findings indicate that a person's body clock has a considerable impact on the immune system. They suggest it's best to do your exercise or training in the evening when you have lower levels of cortisol and a higher rate of saliva flow, which also helps protect against infections.
People returning to exercise and training after an illness or injury should avoid early morning workouts, the authors advise. The same is true for athletes who have increased stress levels because of an upcoming competition and those training at high altitudes. Both of those can depress the immune system.
The findings are reported in the latest issue of the British Journal of Sports Medicine.
The American Council on Exercise has more insight into exercise and the immune system.