The human body is known to go through rhythmic physiological
cycles each day, as the body responds to ambient light, activity
levels, mealtimes, and sleep. Similarly, our immune systems
appear to be affected by the time of day, or by the time of
day that we exercise, based on a recent study in the British
Journal of Sports Medicine.
At 6 a.m., 14 competitive male swimmers performed a 400-meter
crawl five times in a row, with one minute of rest between
each 400-meter set. They repeated the same swim session on
a separate day, but at 6 p.m. After each swim session, saliva
samples were taken from the swimmers and measured for secretions
of cortisol and IgA. Cortisol is a stress hormone that indicates
a weakened immune system; conversely, IgA is a substance that
helps the body fight respiratory tract infections.
Morning swims were associated with the production of significantly
higher levels of cortisol and lower levels of IgA in the swimmers'
bodies. In other words, swimmers who train in the morning
may be at a higher risk for upper respiratory tract infections
than those who train in the evening.
Don't misinterpret these findings, though. They don't necessarily
apply to other sports, and working out in the morning is certainly
better than not working out at all. If your schedule is flexible
enough, however, consider swimming later in the day, rather
than right after you wake up - especially if you're already
feeling "under the weather."
Dimitriou L, Sharp NCC, Doherty M. Circadian effects on the
acute responses of salivary cortisol and IgA in well-trained
swimmers. British Journal of Sports Medicine 2002:36,